LU-14927 scrub: create shared scrub_needs_check() function.

LU-14927 scrub: create shared scrub_needs_check() function.

The current functions osd_consistency_check() in both ldiskfs and
zfs use ktime_* functions which are exported for pure GPL modules.
This is not the case for ZFS. We can refactor the code to create
a new common function scrub_needs_check() that can be used along
side osd_consistency_check(). Fix a few cases where the error
code is not checked for ZFS.

Change-Id: I0cc6cd84a35ecc10b511096f4e749a2961da3bbf
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44689
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>