Linux 6.10 compat: fix rpm-kmod and builtin

The 6.10 kernel broke our rpm-kmod builds.  The 6.10 kernel really
want 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

Fixes: openzfs#16439
Signed-off-by: Tony Hutter <hutter2@llnl.gov>

module/Kbuild.in

@@ -16,8 +16,8 @@ src = @abs_srcdir@
 obj = @abs_builddir@
 else
 zfs_include = $(srctree)/include/zfs
-icp_include = $(srctree)/$(src)/icp/include
-zstd_include = $(srctree)/$(src)/zstd/include
+icp_include = $(src)/icp/include
+zstd_include = $(src)/zstd/include
 ZFS_MODULE_CFLAGS += -include $(zfs_include)/zfs_config.h
 endif
 

rpm/generic/zfs-kmod.spec.in

@@ -145,6 +145,30 @@ for kernel_version in %{?kernel_versions}; do
         %{?kernel_cc} \
         %{?kernel_ld} \
         %{?kernel_llvm}
+
+    # WORKAROUND ALERT
+    #
+    # The 6.10 kernel broke our old way of doing kmod builds.  The old way
+    # was to go into our '_kmod_build_${kernel_version%%___*}' directory, and
+    # then run ../zfs-source-dir/configure from one level down.  This gave
+    # us our Kbuild and Makefile in our '_kmod_build_${kernel_version%%___*}'
+    # directory without any of the source code.  We could then build the modules
+    # using some kind of magic (VPATH?) where we say 'I know our KBuild file is
+    # here, but our source code is actually in this other directory'.  That
+    # doesn't seem to work anymore after the 6.10 kernel due to these commits:
+    #
+    # b1992c3772e6 kbuild: use $(src) instead of $(srctree)/$(src) for source
+    #                      directory
+    #
+    # 9a0ebe5011f4 kbuild: use $(obj)/ instead of $(src)/ for common pattern
+    #                      rules
+    #
+    # The workaround is to just hardlink a copy of the source into our
+    # build directory.
+    #
+    # See https://github.com/openzfs/zfs/issues/16439 for more info.
+    cp -lR ../%{module}-%{version}/module/* module/
+
     make %{?_smp_mflags}
     cd ..
 done