common/rc: add scratch shutdown support for overlayfs

Enable shutdown tests on overlayfs. This requires configuring
overlayfs with TEST|SCRATCH_DEV pointing to block device, which act
as base fs of overlay. The legacy config (pointing TEST|SCRATCH_DEV
to existing directories) won't support shutdown.

Extend _require_scratch_shutdown() to deal with overlay by checking
shutdown support on $OVL_BASE_SCRATCH_MNT instead of $SCRATCH_MNT,
so we check shutdown support status against the underlying upper fs
of overlay.

Introduce new _scratch_shutdown() helper to do the actual shutdown
work and shutdown upper fs in the case of overlay. Also converting
existing bare 'src/godown' calls to use this helper to avoid false
failures when testing overlayfs.

generic/042 and generic/050 assume $SCRATCH_DEV to be a local
device, so add _require_local_device $SCRATCH_DEV rule.

[eguan: rewrite commit log]

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/generic/055

@@ -118,7 +118,7 @@ do
     ls -RF $SCRATCH_MNT >$tmp.ls1
 
     _echofull "godown"
-    src/godown -v -f $SCRATCH_MNT >> $seqres.full
+    _scratch_shutdown -v -f >> $seqres.full
 
     _echofull "unmount"
     _scratch_unmount >>$seqres.full 2>&1 \