Linux 6.10 compat: fix rpm-kmod and builtin

[tonyhutter]

Motivation and Context

Fixes: #16439. This will allow Fedora 39-40 buildbot to work again.

Description

The 6.10 kernel broke our rpm-kmod builds. The 6.10 kernel really wants the source files in the same directory as the object files. This workaround makes rpm-kmod work again. It also updates the builtin kernel codepath to work correctly with 6.10.

See kernel commits:

b1992c3772e6 kbuild: use $(src) instead of $(srctree)/$(src) for source directory
9a0ebe5011f4 kbuild: use $(obj)/ instead of $(src)/ for common pattern rules

Marking as draft until I can test builtin.

How Has This Been Tested?

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.

[tonyhutter]

I started building this PR into the 6.10.5 kernel with copy-builtin, and hit an unrelated bug (#16433). The build did get far enough to build icp and zstd though (this PR changed their build scripts), so I'm going to call that good enough.

[tonyhutter]

Removing WIP tag since I was able to build icp & zstd with builtin using the 6.10.5 kernel.

[behlendorf]

This works for me as a workaround. We may just want to consider switching to always making a copy and not doing a VPATH build.

[tonyhutter]

It turns out my workaround is less a hack than I thought, as kmodtool just copies over the source code into each kernel-build directory

%setup -q -c -T -a 0
# pushd foo-%{version}
# #patch0 -p1
# popd
for kernel_version in %{?kernel_versions} ; do
    cp -a foo-%{version} _kmod_build_${kernel_version%%___*}
done

https://rpmfusion.org/Packaging/KernelModules/Kmods2

I've updated my comment in the code to reflect this.