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

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 +145,24 @@ for kernel_version in %{?kernel_versions}; do
         %{?kernel_cc} \
         %{?kernel_ld} \
         %{?kernel_llvm}
+
+    # Pre-6.10 kernel builds didn't need to copy over the source files to the
+    # build directory.  However we do need to do it though post-6.10 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
+    #
+    # Note that kmodtool actually copies over the source into the build
+    # directory, so what we're doing here is normal.  For efficiency reasons
+    # though we just use hardlinks instead of copying.
+    #
+    # See https://github.com/openzfs/zfs/issues/16439 for more info.
+    cp -lR ../%{module}-%{version}/module/* module/
+
     make %{?_smp_mflags}
     cd ..
 done