ocfs2: Modify removing xattr process for refcount.

The old xattr value remove is quite simple, it just erase the
tree and free the clusters. But as we have added refcount support,
The process is a little complicated.

We have to lock the refcount tree at the beginning, what's more,
we may split the refcount tree in some cases, so meta/credits are
needed.

Signed-off-by: Tao Ma <tao.ma@oracle.com>