ocfs2/dlm: return zero if deref_done message is successfully handled

dlm_deref_lockres_done_handler() should return zero if the message is
successfully handled.

Fixes: 60d663cb

 ("ocfs2/dlm: add DEREF_DONE message").
Signed-off-by: xuejiufei <xuejiufei@huawei.com>
Reviewed-by: Joseph Qi <joseph.qi@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>