ocfs2: Release buffer_head in case of error in ocfs2_double_lock.

In ocfs2_double_lock, when ocfs2_inode_lock for inode1 fails, we
just unlock inode2 and return without releasing buffer we get from
inode_lock(inode2). The good thing is that it is freed by the only
caller ocfs2_rename when it exits.

But I don't think this is a right way for error handling. We should
free the buffer_head we get in ocfs2_double_lock before exit so that
the caller doesn't need to take care of it.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>