[NETNS]: icmp(v6)_sk should not pin a namespace.

So, change icmp(v6)_sk creation/disposal to the scheme used in the
netlink for rtnl, i.e. create a socket in the context of the init_net
and assign the namespace without getting a referrence later.

Also use sk_release_kernel instead of sock_release to properly destroy
such sockets.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>