overlay: fix _scratch_remount with xfs_info 5.0.0

xfs_info version 5.0.0 started using findmnt to find the
filesystem to query. This change resulted in a regression
of _scratch_remount when testing overlay over xfs.
For example, test overlay/035, started to report:
[not run] overlay/035 -- upper fs needs to support d_type

Internally, '_overlay_scratch_mount -o remount' calls
'_supports_filetype $OVL_BASE_SCRATCH_MNT -o remount'
and with the following example mounts:

/dev/vdf /vdf xfs rw,relatime,attr2,inode64,noquota 0 0
/vdf /vdf/ovl-mnt overlay rw,lowerdir=/vdf/lower,upperdir=/vdf/upper...

'_supports_filetype /vdf' returns false and reports:
"/vdf/ovl-mnt: Not on a mounted XFS filesystem".

Regardless of the change in xfs_info, which I proposed a fix
for, there is no reason to test d_type support on remount.
Therefore, fix the regression by skipping unneeded overlayfs
mount logic on remount.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

common/overlay

@@ -105,6 +105,11 @@ _overlay_base_scratch_mount()
 
 _overlay_scratch_mount()
 {
+	if echo "$*" | grep -q remount; then
+		$MOUNT_PROG $SCRATCH_MNT $*
+		return
+	fi
+
 	_overlay_base_scratch_mount && \
 		_overlay_mount $OVL_BASE_SCRATCH_MNT $SCRATCH_MNT $*
 }