ocfs2/dlm: clear migration_pending when migration target goes down

We have found a BUG on res->migration_pending when migrating lock
resources.  The situation is as follows.

dlm_mark_lockres_migration
  res->migration_pending = 1;
  __dlm_lockres_reserve_ast
  dlm_lockres_release_ast returns with res->migration_pending remains
      because other threads reserve asts
  wait dlm_migration_can_proceed returns 1
  >>>>>>> o2hb found that target goes down and remove target
          from domain_map
  dlm_migration_can_proceed returns 1
  dlm_mark_lockres_migrating returns -ESHOTDOWN with
      res->migration_pending still remains.

When reentering dlm_mark_lockres_migrating(), it will trigger the BUG_ON
with res->migration_pending.  So clear migration_pending when target is
down.

Signed-off-by: Jiufei Xue <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>

fs/ocfs2/dlm/dlmmaster.c

@@ -2843,6 +2843,8 @@ again:
 	res->state &= ~DLM_LOCK_RES_BLOCK_DIRTY;
 	if (!ret)
 		BUG_ON(!(res->state & DLM_LOCK_RES_MIGRATING));
+	else
+		res->migration_pending = 0;
 	spin_unlock(&res->spinlock);
 
 	/*