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>