moved kernel_write out of splice translation unit

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.
/dev/mem:: support compiling out splice

Prepares /dev/mem for the upcoming absence of splice file_operations when
CONFIG_SYSCALL_SPLICE is undefined.
virtio: support compiling out splice

Prepares virtio console for the upcoming absence of splice file_operations when
CONFIG_SYSCALL_SPLICE is undefined.
fs: Macros to define splice file_operations

Provides a CONFIG_SYSCALL_SPLICE compatible way of defining the .splice_read
and .splice_write file_operations so that they can later be compiled out when
the kernel is configured without the splice-family syscalls
mm/shmem: support compiling out splice

Prepares shmem for the upcoming absence of splice file_operations when
CONFIG_SYSCALL_SPLICE is undefined.
net/ipv4: support compiling out splice

Prepares ipv4 for the upcoming absence of splice file_operations when
CONFIG_SYSCALL_SPLICE is undefined.
net/ipv6: support compiling out splice

Prepares ipv6 for the upcoming absence of splice file_operations when
CONFIG_SYSCALL_SPLICE is undefined.
net/socket: support compiling out splice

Prepares socket for the upcoming absence of splice file_operations when
CONFIG_SYSCALL_SPLICE is undefined.
kernel/trace: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
kernel/relay: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
mm/shmem: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
fs/xfs: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
fs/ocfs: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
fs/nfsd: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
fs/nfs: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
fs/fuse: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
fs/code: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
drivers/char/virtio: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
drivers/char/mem: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).
net.ipv4: optionally compile out splice

Compile out splice-related code when system doesn't support splice family of
syscalls (i.e. CONFIG_SYSCALL_SPLICE is undefined).