fs: move sendfile syscall into fs/splice

sendfile functionally forms part of the splice group of syscalls (splice,
vmsplice and tee). Grouping sendfile with splice paves the way to compiling out
the splice group of syscalls for embedded systems that do not need these.

add/remove: 0/0 grow/shrink: 7/2 up/down: 86/-61 (25)
function                                     old     new   delta
file_start_write                              34      68     +34
file_end_write                                29      58     +29
sys_pwritev                                  115     122      +7
sys_preadv                                   115     122      +7
fdput_pos                                     29      36      +7
sys_pwrite64                                 115     116      +1
sys_pread64                                  115     116      +1
sys_tee                                      497     491      -6
sys_splice                                  1075    1020     -55
fs: moved kernel_write to fs/read_write

kernel_write shares infrastructure with the read_write translation unit but not
with the splice translation unit. Grouping kernel_write with the read_write
translation unit is more logical. It also paves the way to compiling out the
splice group of syscalls for embedded systems that do not need them.
fs/splice: support compiling out splice-family syscalls

Many embedded systems will not need the splice-family syscalls (splice,
vmsplice, tee and sendfile). Omitting them saves space.  This adds a new EXPERT
config option CONFIG_SYSCALL_SPLICE (default y) to support compiling them out.

The goal is to completely compile out fs/splice. To achieve this, the remaining
patch-set will deal with fs/splice exports. As far as possible, the impact on
other device drivers will be minimized so as to reduce the maintenance burden
of CONFIG_SYSCALL_SPLICE.

The use of exported functions will be solved by transparently mocking them out
with static inlines. Use of the exported pipe_buf_operations struct however
require direct modification in fs/fuse and net/core. The next two patches will
deal with this.

Once all exports are solved, fs/splice can be compiled out.

The bloat benefit of this patch given a tinyconfig is:

add/remove: 0/16 grow/shrink: 2/5 up/down: 114/-3693 (-3579)
function                                     old     new   delta
splice_direct_to_actor                       348     416     +68
splice_to_pipe                               371     417     +46
splice_from_pipe_next                        107     106      -1
fdput                                         11       -     -11
signal_pending                                39      26     -13
fdget                                         56      42     -14
user_page_pipe_buf_ops                        20       -     -20
user_page_pipe_buf_steal                      25       -     -25
file_end_write                                58      29     -29
file_start_write                              68      34     -34
pipe_to_user                                  43       -     -43
wakeup_pipe_readers                           54       -     -54
do_splice_to                                  87       -     -87
ipipe_prep.part                               92       -     -92
opipe_prep.part                              119       -    -119
sys_sendfile                                 122       -    -122
sys_sendfile64                               126       -    -126
sys_vmsplice                                 137       -    -137
vmsplice_to_user                             205       -    -205
sys_tee                                      491       -    -491
do_sendfile                                  492       -    -492
vmsplice_to_pipe                             558       -    -558
sys_splice                                  1020       -   -1020
fs/fuse: support compiling out splice

To implement splice support, fs/fuse makes use of nosteal_pipe_buf_ops. This
struct is exported by fs/splice. The goal of the larger patch set is to
completely compile out fs/splice, so uses of the exported struct need to be
compiled out along with fs/splice.

This patch therefore compiles out splice support in fs/fuse when
CONFIG_SYSCALL_SPLICE is undefined.
net/core: support compiling out splice

To implement splice support, net/core makes use of nosteal_pipe_buf_ops. This
struct is exported by fs/splice. The goal of the larger patch set is to
completely compile out fs/splice, so uses of the exported struct need to be
compiled out along with fs/splice.

This patch therefore compiles out splice support in net/core when
CONFIG_SYSCALL_SPLICE is undefined. The compiled out function skb_splice_bits
is transparently mocked out with a static inline. The greater patch set removes
userspace splice support so it cannot be called anyway.
fs/splice: full support for compiling out splice

Entirely compile out splice translation unit when the system is configured
without splice family of syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).

Exported fs/splice functions are transparently mocked out with static inlines.
Because userspace support for splice has already been removed by this
patch-set, the exported functions cannot be called anyway. Mocking them out
prevents a maintenance burden on file system drivers.

The bloat score resulting from this patch given a tinyconfig is:
add/remove: 0/25 grow/shrink: 0/5 up/down: 0/-4845 (-4845)
function                                     old     new   delta
pipe_to_null                                   4       -      -4
generic_pipe_buf_nosteal                       6       -      -6
spd_release_page                              10       -     -10
PageUptodate                                  22      11     -11
lock_page                                     36      24     -12
page_cache_pipe_buf_release                   16       -     -16
splice_write_null                             24       4     -20
page_cache_pipe_buf_ops                       20       -     -20
nosteal_pipe_buf_ops                          20       -     -20
default_pipe_buf_ops                          20       -     -20
generic_splice_sendpage                       24       -     -24
splice_shrink_spd                             27       -     -27
direct_splice_actor                           47       -     -47
default_file_splice_write                     49       -     -49
wakeup_pipe_writers                           54       -     -54
write_pipe_buf                                71       -     -71
page_cache_pipe_buf_confirm                   80       -     -80
splice_grow_spd                               87       -     -87
splice_from_pipe                              93       -     -93
splice_from_pipe_next                        106       -    -106
pipe_to_sendpage                             109       -    -109
page_cache_pipe_buf_steal                    114       -    -114
generic_file_splice_read                     131       8    -123
do_splice_direct                             148       -    -148
__splice_from_pipe                           246       -    -246
splice_direct_to_actor                       416       -    -416
splice_to_pipe                               417       -    -417
default_file_splice_read                     688       -    -688
iter_file_splice_write                       702       4    -698
__generic_file_splice_read                  1109       -   -1109

The bloat score for the entire CONFIG_SYSCALL_SPLICE patch-set is:
add/remove: 0/41 grow/shrink: 5/7 up/down: 23/-8422 (-8399)
function                                     old     new   delta
sys_pwritev                                  115     122      +7
sys_preadv                                   115     122      +7
fdput_pos                                     29      36      +7
sys_pwrite64                                 115     116      +1
sys_pread64                                  115     116      +1
pipe_to_null                                   4       -      -4
generic_pipe_buf_nosteal                       6       -      -6
spd_release_page                              10       -     -10
fdput                                         11       -     -11
PageUptodate                                  22      11     -11
lock_page                                     36      24     -12
signal_pending                                39      26     -13
fdget                                         56      42     -14
page_cache_pipe_buf_release                   16       -     -16
user_page_pipe_buf_ops                        20       -     -20
splice_write_null                             24       4     -20
page_cache_pipe_buf_ops                       20       -     -20
nosteal_pipe_buf_ops                          20       -     -20
default_pipe_buf_ops                          20       -     -20
generic_splice_sendpage                       24       -     -24
user_page_pipe_buf_steal                      25       -     -25
splice_shrink_spd                             27       -     -27
pipe_to_user                                  43       -     -43
direct_splice_actor                           47       -     -47
default_file_splice_write                     49       -     -49
wakeup_pipe_writers                           54       -     -54
wakeup_pipe_readers                           54       -     -54
write_pipe_buf                                71       -     -71
page_cache_pipe_buf_confirm                   80       -     -80
splice_grow_spd                               87       -     -87
do_splice_to                                  87       -     -87
ipipe_prep.part                               92       -     -92
splice_from_pipe                              93       -     -93
splice_from_pipe_next                        107       -    -107
pipe_to_sendpage                             109       -    -109
page_cache_pipe_buf_steal                    114       -    -114
opipe_prep.part                              119       -    -119
sys_sendfile                                 122       -    -122
generic_file_splice_read                     131       8    -123
sys_sendfile64                               126       -    -126
sys_vmsplice                                 137       -    -137
do_splice_direct                             148       -    -148
vmsplice_to_user                             205       -    -205
__splice_from_pipe                           246       -    -246
splice_direct_to_actor                       348       -    -348
splice_to_pipe                               371       -    -371
do_sendfile                                  492       -    -492
sys_tee                                      497       -    -497
vmsplice_to_pipe                             558       -    -558
default_file_splice_read                     688       -    -688
iter_file_splice_write                       702       4    -698
sys_splice                                  1075       -   -1075
__generic_file_splice_read                  1109       -   -1109