Linux 6.10 compat: fix rpm-kmod and builtin

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

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #16439
Closes #16450