common: use _scratch_mount helper in _require_relatime()

Change the way how _require_relatime() mount $SCRATCH_DEV, use
_scratch_mount helper so $SCRATCH_DEV is mounted with selinux context,
to avoid "same superblock, different selinux context" failure.

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/rc

@@ -2399,7 +2399,7 @@ _require_atime()
 _require_relatime()
 {
         _scratch_mkfs > /dev/null 2>&1
-        _mount -t $FSTYP -o relatime $SCRATCH_DEV $SCRATCH_MNT || \
+        _scratch_mount -o relatime || \
                 _notrun "relatime not supported by the current kernel"
 	_scratch_unmount
 }