common/xfs: refactor xfs_scrub presence testing

common/xfs: refactor xfs_scrub presence testing

Move all the requirements checking for xfs_scrub into a helper function.
Make sure the helper properly detects the presence of the scrub ioctl
and situations where we can't run scrub (e.g. norecovery).

Refactor the existing three xfs_scrub call sites to use the helper to
check if it's appropriate to run scrub.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>