ocfs2/dlm: return appropriate value when dlm_grab() returns NULL

dlm_grab() may return NULL when the node is doing unmount.  When doing
code review, we found that some dlm handlers may return error to caller
when dlm_grab() returns NULL and make caller BUG or other problems.
Here is an example:

Node 1                                 Node 2
receives migration message
from node 3, and send
migrate request to others
                                     start unmounting

                                     receives migrate request
                                     from node 1 and call
                                     dlm_migrate_request_handler()

                                     unmount thread unregisters
                                     domain handlers and removes
                                     dlm_context from dlm_domains

                                     dlm_migrate_request_handlers()
                                     returns -EINVAL to node 1
Exit migration neither clearing the
migration state nor sending
assert master message to node 3 which
cause node 3 hung.
Signed-off-by: Jiufei Xue <xuejiufei@huawei.com>
Reviewed-by: Joseph Qi <joseph.qi@huawei.com>
Reviewed-by: Yiwen Jiang <jiangyiwen@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

@@ -3050,7 +3050,7 @@ int dlm_migrate_request_handler(struct o2net_msg *msg, u32 len, void *data,
 	int ret = 0;
 
 	if (!dlm_grab(dlm))
-		return -EINVAL;
+		return 0;
 
 	name = migrate->name;
 	namelen = migrate->namelen;

fs/ocfs2/dlm/dlmunlock.c

@@ -421,7 +421,7 @@ int dlm_unlock_lock_handler(struct o2net_msg *msg, u32 len, void *data,
 	}
 
 	if (!dlm_grab(dlm))
-		return DLM_REJECTED;
+		return DLM_FORWARD;
 
 	mlog_bug_on_msg(!dlm_domain_fully_joined(dlm),
 			"Domain %s not fully joined!\n", dlm->name);