inet: add struct net argument to inet_lhashfn

Listening-on-one-port sockets in many namespaces produce long 
chains in the listening_hash-es, so prepare the inet_lhashfn to 
take struct net into account.

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