LU-12634 build: kbuild changes in 5.3 drop subdir-m

LU-12634 build: kbuild changes in 5.3 drop subdir-m

Several changes in kbuild affect the way external modules
can be built. In the Linux 5.3-rc4 series subdir-m has
been removed.

Linux commit: c07d8d47bca1b325102fa2be3a463075f7b051d9

Test-Parameters: trivial
Cray-bug-id: LUS-7689
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: Id1f248ac4ccdee8d2a2d177b4fdff4444d2084d1
Reviewed-on: https://review.whamcloud.com/35786
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>