LU-12477 libcfs: Further reduce complexity for shrinkers.

LU-12477 libcfs: Further reduce  complexity for shrinkers.

Commit c4c17fa4a3f5 ("LU-12477 libcfs: Remove obsolete config checks")
reduced the complexity of shinkers by removing support for older
kernels, but could have gone a lot further.  This patch adds
further simplification.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ibcc84f61e542b503f795b16a7144e430f8b73582
Reviewed-on: https://review.whamcloud.com/40831
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>