ipv6: try not to take rtnl_lock in ip6mr_sk_done

Avoid traversing the list of mr6_tables (which requires the
rtnl_lock) in ip6mr_sk_done(), when we know in advance that
a match will not be found.
This can happen when rawv6_close()/ip6mr_sk_done() is invoked
on non-mroute6 sockets.
This patch helps reduce rtnl_lock contention when destroying
a large number of net namespaces, each having a non-mroute6
raw socket.

v2: same patch, only fixed subject line and expanded comment.
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6mr.c

@@ -1617,6 +1617,10 @@ int ip6mr_sk_done(struct sock *sk)
 	struct net *net = sock_net(sk);
 	struct mr6_table *mrt;
 
+	if (sk->sk_type != SOCK_RAW ||
+	    inet_sk(sk)->inet_num != IPPROTO_ICMPV6)
+		return err;
+
 	rtnl_lock();
 	ip6mr_for_each_table(mrt, net) {
 		if (sk == mrt->mroute6_sk) {