net: add a way to get a network namespace for a socket

Each sockets belongs to one network namespace and operates
in this network namespace.

socket_diag reports informations about sockets from
one network namespace, but it doesn't report sockets which
are not bound or connected to somewhere. So we need to have
a way to get network namespaces for such sockets.

Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>