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

Function 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.

---
 fs/ocfs2/dlm/dlmmaster.c | 2 +-
 fs/ocfs2/dlm/dlmunlock.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

index ce38b4c..c43da7f 100644
@@ -3048,7 +3048,7 @@ 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;

index 2e3c9db..1082b2c 100644
@@ -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);

1.8.4.3