Don't build packages that haven't been selected.

Currently, when configure --with-config is used, selective compilation
is only effective for the simple "make" case. Package builders (e.g.
make rpm) still build everything (utils and modules). This patch fixes
that.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851
Make configure builtin-aware.

This patch adds a new option to configure: --enable-linux-builtin. When
this option is used, the following happens:

 - Compilation of kernel modules is disabled.

 - A failure to find UTS_RELEASE is followed by a suggestion to run
   "make prepare" on the kernel source tree.

This patch also adds a new test which tries to compile an empty module
as a basic toolchain sanity test. If it fails and the option was
specified, the error is followed by a suggestion to run "make scripts"
on the kernel source tree.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851
Fake modpost stage for LINUX_COMPILE.

Currently, when building a test case, we're compiling an entire Linux
module from beginning to end. This includes the MODPOST stage, which
generates a "conftest.mod.c" file with some boilerplate module
declaration code.

This poses a problem when configuring for built-in on kernels which have
loadable module support disabled. In this case conftest.mod.c is
referencing disabled code, resulting in a compilation failure, thus
breaking the tests.

This patch fixes the issue by faking the modpost stage when the
--enable-linux-builtin option is provided.  It does so by forcing the
modpost command to be /bin/true, and using an empty conftest.mod.c file.
The test module still compiles fine, although the result isn't loadable,
but we don't really care at this point.

Note it is important to preserve the modpost stage when building out of
tree.  The ZFS_AC_KERNEL_BLK_END_REQUEST, ZFS_AC_KERNEL_BLK_QUEUE_FLUSH,
and ZFS_AC_KERNEL_BLK_RQ_BYTES configure checks all depend on it to
identify GPL-only symbols.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851
When checking for symbol exports, try compiling.

This patch adds a new autoconf function: ZFS_LINUX_TRY_COMPILE_SYMBOL.
This new function does the following:

 - Call LINUX_TRY_COMPILE with the specified parameters.
 - If unsuccessful, return false.
 - If successful and we're configuring with --enable-linux-builtin,
   return true.
 - Else, call CHECK_SYMBOL_EXPORT with the specified parameters and
   return the result.

All calls to CHECK_SYMBOL_EXPORT are converted to
LINUX_TRY_COMPILE_SYMBOL so that the tests work even when configuring
for builtin on a kernel which doesn't have loadable module support, or
hasn't been built yet.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851
Add script for builtin module building.

This commit introduces a "copy-builtin" script designed to prepare a
kernel source tree for building ZFS as a builtin module. The script
makes a full copy of all needed files, thus making the kernel source
tree fully independent of the zfs source package.

To achieve that, some compilation flags (-include, -I) have been moved
to module/Makefile. This Makefile is only used when compiling external
modules; when compiling builtin modules, a Kbuild file generated by the
configure-builtin script is used instead. This makes sure Makefiles
inside the kernel source tree does not contain references to the zfs
source package.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851
Use /sys/module instead of /proc/modules.

When libzfs checks if the module is loaded or not, it currently reads
/proc/modules and searches for a line matching the module name.

Unfortunately, if the module is included in the kernel itself (built-in
module), then /proc/modules won't list it, so libzfs will wrongly conclude
that the module is not loaded, thus making all ZFS userspace tools unusable.

Fortunately, all loaded modules appear as directories in /sys/module, even
built-in ones. Thus we can use /sys/module in lieu of /proc/modules to fix
the issue.

As a bonus, the code for checking becomes much simpler.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851
Merge branch 'builtin-clean'

Support in-tree builtin module building.

These commits add support for compiling the ZFS module as a built-in
kernel module by copying the module code into the kernel source tree.
Here's the procedure:

  - Create your kernel configuration (`.config` file) as usual. This
    has to be done first so that ZFS's configure script is able to
    detect kernel features correctly.
  - Run `make prepare scripts` inside the kernel source tree.
  - Run `./configure --enable-linux-builtin --with-linux=/usr/src/linux-...`
    inside the ZFS directory.
  - Run `./copy-builtin /usr/src/linux-...` inside the ZFS directory.
  - In the kernel source tree, enable the `CONFIG_ZFS` option (e.g. using
    `make menuconfig`). Note that this option depends on `CONFIG_SPL`
    (see zfsonlinux/spl@744038069d3dc65e721b5b8cc5c37d8c7fcbd8c0).
  - Build the kernel as usual.

ZFS module parameters can be set at boot time using the following syntax
on the kernel command line: `zfs.parameter_name=parameter_value`.

Note that you also need to rebuild the userspace tools (see
zfsonlinux/zfs@f09398cec665259a4c2f96726680fbd3b0a3bac3).

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851