xfrm: Set transport header to fix UDP GRO handling

The referenced commit replaced a call to __xfrm4|6_udp_encap_rcv() with
a custom check for non-ESP markers. But what the called function also
did was setting the transport header to the ESP header. The function
that follows, esp4|6_gro_receive(), relies on that being set when it calls
xfrm_parse_spi(). We have to set the full offset as the skb's head was
not moved yet so adding just the UDP header length won't work.

Fixes: e3fd05777685 ("xfrm: Fix UDP GRO handling for some corner cases")
Signed-off-by: Tobias Brunner <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/ipv4/xfrm4_input.c

···
 	if (len <= sizeof(struct ip_esp_hdr) || udpdata32[0] == 0)
 		goto out;
 
+	/* set the transport header to ESP */
+	skb_set_transport_header(skb, offset);
+
 	NAPI_GRO_CB(skb)->proto = IPPROTO_UDP;
 
 	pp = call_gro_receive(ops->callbacks.gro_receive, head, skb);

net/ipv6/xfrm6_input.c

···
 	if (len <= sizeof(struct ip_esp_hdr) || udpdata32[0] == 0)
 		goto out;
 
+	/* set the transport header to ESP */
+	skb_set_transport_header(skb, offset);
+
 	NAPI_GRO_CB(skb)->proto = IPPROTO_UDP;
 
 	pp = call_gro_receive(ops->callbacks.gro_receive, head, skb);