If master migrate this lock resource to node when it happened to purge
it, a new lock resource will be created and inserted into hash list.  If
then master goes down, the lock resource being purged is recovered, so
there exist two lock resource with different owner.  So return error to
master if the lock resource is in DROPPING state, master will retry to
migrate this lock resource.

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>