xfrm: fix inbound ipv4/udp/esp packets to UDPv6 dualstack sockets

Before Linux v5.8 an AF_INET6 SOCK_DGRAM (udp/udplite) socket
with SOL_UDP, UDP_ENCAP, UDP_ENCAP_ESPINUDP{,_NON_IKE} enabled
would just unconditionally use xfrm4_udp_encap_rcv(), afterwards
such a socket would use the newly added xfrm6_udp_encap_rcv()
which only handles IPv6 packets.

Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Fixes: 0146dca ('xfrm: add support for UDPv6 encapsulation of ESP')
Signed-off-by: Maciej Żenczykowski <maze@google.com>

net/ipv6/xfrm6_input.c

@@ -86,6 +86,9 @@ int xfrm6_udp_encap_rcv(struct sock *sk, struct sk_buff *skb)
 	__be32 *udpdata32;
 	__u16 encap_type = up->encap_type;
 
+	if (skb->protocol == htons(ETH_P_IP))
+		xfrm4_udp_encap_rcv(sk, skb);
+
 	/* if this is not encapsulated socket, then just return now */
 	if (!encap_type)
 		return 1;