xfs: convert scrub to use mount-based feature checks

The scrub feature checks are the last place that the superblock
feature checks are used. Convert them to mount based feature checks.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>