xfs/504: Add scratch_mount before checking for xfs_scrub unicode support

xfs/504: Add scratch_mount before checking for xfs_scrub unicode support

We may not detect the error `Inappropriate ioctl for device`, while running
`$XFS_IO_PROG -c "scrub probe" "$mountpoint"`, if scratch device is not
mounted before invoking _check_xfs_scrub_does_unicode(). So do
_scratch_mount before checking for xfs_scrub support.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>