ocfs2/dlm: clear DROPPING_REF flag when the master goes down

If the master goes down after return in-progress for deref message.  The
lock resource on non-master node can not be purged.  Clear the
DROPPING_REF flag and recovery it.

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