udp: add missing WRITE_ONCE() around up->encap_rcv

UDP_ENCAP_ESPINUDP_NON_IKE setsockopt() writes over up->encap_rcv
while other cpus read it.

Fixes: 067b207b ("[UDP]: Cleanup UDP encapsulation code")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>