xfs_scrub: classify lack of ioctl support as a runtime error

If the kernel doesn't have the SCRUB_METADATA ioctl that's a runtime
error, not a fs error.  Account it as such.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>