[NETNS][RAW]: Eliminate explicit init_net references.

Happily, in all the rest places (->bind callbacks only), that require the
struct net, we have a socket, so get the net from it.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>