generic: test orphan inode recovery on RO mount

generic: test orphan inode recovery on RO mount

There's a bug in xfs where the orphan inode list
is not processed on a readonly mount, and is still not
processed even if it gets mounted rw.  This test exercises
that, and makes sure that the unlinked list is empty after
a transition to rw and an unmount.

While we're at it, sanity check the same thing for pure ro
and rw unmounts.

Now that shutdown is generic, we can test other filesystems
as well, so this is in the generic group.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>