autoconf: Pretend `CONFIG_MODULES` is always on
- Unconditionally inject `CONFIG_MODULES` make variable
and `#define CONFIG_MODULES` to Kbuild in `ZFS_LINUX_COMPILE`
autoconf function to emulate loadable kernel modules support.
This allows OpenZFS to perform Linux checks despite
`CONFIG_MODULES=n` in the actual Linux config.
- Add `ZFS_AC_KERNEL_CONFIG_MODULES` check which encompasses
the logic from `ZFS_AC_KERNEL_TEST_MODULE` with additional
diagnostic messages to the user
- Removed `ZFS_AC_KERNEL_TEST_MODULE` as it merely duplicates
every check in `ZFS_AC_KERNEL_CONFIG_DEFINED`
- Moved `ZFS_AC_MODULE_SYMVERS` after `ZFS_AC_KERNEL_CONFIG_DEFINED`
so the user has a chance to see the proper diagnostic from the
steps before.
A workaround for Linux's
```
commit
3e3005df73b535cb849cf4ec8075d6aa3c460f68
Author: Masahiro Yamada <masahiroy@kernel.org>
Date: Wed Mar 31 22:38:03 2021 +0900
kbuild: unify modules(_install) for in-tree and external modules
If you attempt to build or install modules ('make modules(_install)'
with CONFIG_MODULES disabled, you will get a clear error message, but
nothing for external module builds.
Factor out the modules and modules_install rules into the common part,
so you will get the same error message when you try to build external
modules with CONFIG_MODULES=n.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
```
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #10832
Closes #13361