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>

net/ipv4/udp.c

@@ -2675,10 +2675,12 @@ int udp_lib_setsockopt(struct sock *sk, int level, int optname,
 		case UDP_ENCAP_ESPINUDP_NON_IKE:
 #if IS_ENABLED(CONFIG_IPV6)
 			if (sk->sk_family == AF_INET6)
-				up->encap_rcv = ipv6_stub->xfrm6_udp_encap_rcv;
+				WRITE_ONCE(up->encap_rcv,
+					   ipv6_stub->xfrm6_udp_encap_rcv);
 			else
 #endif
-				up->encap_rcv = xfrm4_udp_encap_rcv;
+				WRITE_ONCE(up->encap_rcv,
+					   xfrm4_udp_encap_rcv);
 #endif
 			fallthrough;
 		case UDP_ENCAP_L2TPINUDP: