[NETNS]: Make icmpv6_sk per namespace.

All preparations are done. Now just add a hook to perform an
initialization on namespace startup and replace icmpv6_sk macro with
proper inline call.  Actual namespace the packet belongs too will be
passed later along with the one for the routing.

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>