common/rc: introduce _overlay_mount_dirs helper

Introduce a new _overlay_mount_dirs() helper to do the actual
overlay mount operation and factor _overlay_scratch_mount and
_overlay_mount code to use a single implemention of
_overlay_mount_dirs(). Also convert all bare mount of overlay in
tests to use it.

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

tests/overlay/005

@@ -93,10 +93,7 @@ $XFS_IO_PROG -f -c "truncate 48m" ${lowerd}/test_file \
 	>>$seqres.full 2>&1
 
 # mount new overlayfs
-$MOUNT_PROG -t overlay $SELINUX_MOUNT_OPTIONS \
-	    -o lowerdir=$lowerd -o upperdir=$upperd \
-	    -o workdir=$workd \
-	    $SCRATCH_DEV $SCRATCH_MNT
+_overlay_mount_dirs $lowerd $upperd $workd $SCRATCH_DEV $SCRATCH_MNT
 
 # the open call triggers copy-up and it will fail ENOSPC
 $XFS_IO_PROG -f -c "o" ${SCRATCH_MNT}/test_file \

tests/overlay/010

@@ -67,9 +67,8 @@ touch $lowerdir1/testdir/a $lowerdir1/testdir/b
 mknod $lowerdir2/testdir/a c 0 0
 
 # Mount overlayfs and remove testdir, which led to kernel crash
-$MOUNT_PROG -t overlay -o lowerdir=$lowerdir2:$lowerdir1 \
-	    -o upperdir=$upperdir,workdir=$workdir \
-	    $SCRATCH_DEV $SCRATCH_MNT
+_overlay_mount_dirs "$lowerdir2:$lowerdir1" $upperdir $workdir \
+		    $SCRATCH_DEV $SCRATCH_MNT
 rm -rf $SCRATCH_MNT/testdir
 
 # success, all done