udp: gro behind static key

Avoid the socket lookup cost in udp_gro_receive if no socket has a
udp tunnel callback configured.

udp_sk(sk)->gro_receive requires a registration with
setup_udp_tunnel_sock, which enables the static key.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>