ip6mr: ip6mr_sk_done() can exit early in common cases

In many cases, ip6mr_sk_done() is called while no ipmr socket
has been registered.

This removes 4 rtnl acquisitions per netns dismantle,
with following callers:

igmp6_net_exit(), tcpv6_net_exit(), ndisc_net_exit()
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6mr.c

@@ -1575,6 +1575,9 @@ int ip6mr_sk_done(struct sock *sk)
 	    inet_sk(sk)->inet_num != IPPROTO_ICMPV6)
 		return err;
 
+	if (!atomic_read(&net->ipv6.devconf_all->mc_forwarding))
+		return err;
+
 	rtnl_lock();
 	ip6mr_for_each_table(mrt, net) {
 		if (sk == rtnl_dereference(mrt->mroute_sk)) {