ocfs2/dlm: solve a BUG when deref failed in dlm_drop_lockres_ref

We found a BUG situation that lockres is migrated during deref described
below.  To solve the BUG, we could purge lockres directly when other
node says I did not have a ref.  Additionally, we'd better purge lockres
if master goes down, as no one will response deref done.

Node 1                  Node 2(old master)             Node3(new master)
dlm_purge_lockres
send deref to N2

                        leave domain
                        migrate lockres to N3
                                                       finish migration
                                                       send do assert
                                                       master to N1

receive do assert msg
form N3, but can not
find lockres because
DROPPING_REF is set,
so the owner is still
N2.

                        receive deref from N1
                        and response -EINVAL
                        because lockres is migrated

BUG when receive -EINVAL
in dlm_drop_lockres_ref

Fixes: 842b90b6

 ("ocfs2/dlm: return in progress if master can not clear the refmap bit right now")

Link: http://lkml.kernel.org/r/57845103.3070406@huawei.com
Signed-off-by: Jun Piao <piaojun@huawei.com>
Reviewed-by: Joseph Qi <joseph.qi@huawei.com>
Reviewed-by: Jiufei Xue <xuejiufei@huawei.com>
Reviewed-by: 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>