netfilter: nfnetlink: fix potential dead lock in nfnetlink_rcv_msg()

When type is NFNL_CB_MUTEX and -EAGAIN error occur in nfnetlink_rcv_msg(),
it does not execute nfnl_unlock(). That would trigger potential dead lock.

Fixes: 50f2db9e ("netfilter: nfnetlink: consolidate callback types")
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nfnetlink.c

@@ -294,6 +294,7 @@ static int nfnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			nfnl_lock(subsys_id);
 			if (nfnl_dereference_protected(subsys_id) != ss ||
 			    nfnetlink_find_client(type, ss) != nc) {
+				nfnl_unlock(subsys_id);
 				err = -EAGAIN;
 				break;
 			}