linux: Fix out-of-src builds

The linux kernel modules haven't been building successfully when the
build occurs in a separate directory than the source code, which is a
common build pattern in Linux. Was not able to determine the root cause,
but the %.o targets in subdirectories are no longer being matched by the
pattern targets in the Linux Kbuild system. This change fixes the issue
by dynamically creating the missing ones inside our Kbuild.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes #17517