Fix Linux kernel directories detection

[szubersk]

Motivation and Context

Close #9935

Description

Most modern Linux distributions have separate locations for bare
source (source) and prebuilt (build) files. Additionally, there are
source and build symlinks in /lib/modules/$(KERNEL_VERSION)
pointing to them. The directory search order is now:

• configure command line values if both --with-linux and
  --with-linux-obj were defined

• If only --with-linux was defined, --with-linux-obj is assumed
  to have the same value as --with-linux

• If neither --with-linux nor --with-linux-obj were defined
  autodetection is used:

  • /lib/modules/$(uname -r)/{source,build} respectively, if exist.

  • If only /lib/modules/$(uname -r)/build exists, it is assumed
    to be both source and build directory.

  • The first directory in /lib/modules with the highest version
    number according to sort -V which contains both source and
    build symlinks/directories. If module directory contains only
    build component, it is assumed to be both source and build
    directory.

  • Last resort: the first directory matching /usr/src/kernels/*
    and /usr/src/linux-* with the highest version number according
    to sort -V is assumed to be both source and build directory.

This fix should be cherry-picked to zfs-0.8-release and zfs-2.0-release branches with high priority as it fixes build problems in the upstream.

How Has This Been Tested?

make -j1 deb-utils deb-dkms on

• zfs-0.8-release + vanilla 5.9.16 kernel
• zfs-2.0-release + Debian bullseye kernel (5.10 + patches)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[szubersk]

Now it's ready-ready.

[behlendorf]

Where each distribution locates its kernel headers and objects has changed somewhat since these checks were originally written, so thanks for revisiting this logic in the context of issue 9935. What's proposed here looks good, and it's really nice to see a human readable explanation of the detection logic. Let's add it to the comment right before ZFS_AC_KERNEL.

[szubersk]

Where each distribution locates its kernel headers and objects has changed somewhat since these checks were originally written, so thanks for revisiting this logic in the context of issue 9935. What's proposed here looks good, and it's really nice to see a human readable explanation of the detection logic. Let's add it to the comment right before ZFS_AC_KERNEL.

Understood. I hit a lot of quirks myself while testing this PR. I hope all Linux distros are going to switch to /lib/modules/$(KERNEL_VERSION)/{source,build} convention (hello Ubuntu).