esp4: prepare esp_input_done2 for non-UDP encapsulation

For espintcp encapsulation, we will need to get the source port from the
TCP header instead of UDP. Introduce a variable to hold the port.

Co-developed-by: Herbert Xu <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>
Signed-off-by: Sabrina Dubroca <[e-mail]>
Acked-by: David S. Miller <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/ipv4/esp4.c

···
 	if (x->encap) {
 		struct xfrm_encap_tmpl *encap = x->encap;
 		struct udphdr *uh = (void *)(skb_network_header(skb) + ihl);
+		__be16 source;
+
+		switch (x->encap->encap_type) {
+		case UDP_ENCAP_ESPINUDP:
+		case UDP_ENCAP_ESPINUDP_NON_IKE:
+			source = uh->source;
+			break;
+		default:
+			WARN_ON_ONCE(1);
+			err = -EINVAL;
+			goto out;
+		}
 
 		/*
 		 * 1) if the NAT-T peer's IP or port changed then
···
 		 *    SRC ports.
 		 */
 		if (iph->saddr != x->props.saddr.a4 ||
-		    uh->source != encap->encap_sport) {
+		    source != encap->encap_sport) {
 			xfrm_address_t ipaddr;
 
 			ipaddr.a4 = iph->saddr;
-			km_new_mapping(x, &ipaddr, uh->source);
+			km_new_mapping(x, &ipaddr, source);
 
 			/* XXX: perhaps add an extra
 			 * policy check here, to see