overlay: add filesystem check helper

Add filesystem check helper for the upcoming fsck.overlay utility.
This helper invoke fsck.overlay to check and optionally repair
underlying directories of overlay filesystem.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

common/overlay

@@ -168,3 +168,17 @@ _require_scratch_overlay_features()
 
 	_scratch_unmount
 }
+
+# Helper function to check underlying dirs of overlay filesystem
+_overlay_fsck_dirs()
+{
+	local lowerdir=$1
+	local upperdir=$2
+	local workdir=$3
+	shift 3
+
+	[[ ! -x "$FSCK_OVERLAY_PROG" ]] && return 0
+
+	$FSCK_OVERLAY_PROG -o lowerdir=$lowerdir -o upperdir=$upperdir \
+			   -o workdir=$workdir $*
+}