net: save network namespaces for sockets

Each socket has to be restored in a proper namespaces where
it has been created.

Here is an issue about unconnected and unbound sockets,
they are not reported via socket-diag and we can't to
get their network namespaces.

v2: add a comment before get_socket_ns()
    remove nsid from sk_packet_entry

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