ocfs2: Attach xattr clusters to refcount tree.

In ocfs2, when xattr's value is larger than OCFS2_XATTR_INLINE_SIZE,
it will be kept outside of the blocks we store xattr entry. And they
are stored in a b-tree also. So this patch try to attach all these
clusters to refcount tree also.

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