net: Reset encap_level to avoid resetting features on inner IP headers

This patch corrects an oversight in which we were allowing the encap_level
value to pass from the outer headers to the inner headers.  As a result we
were incorrectly identifying UDP or GRE tunnels as also making use of ipip
or sit when the second header actually represented a tunnel encapsulated in
either a UDP or GRE tunnel which already had the features masked.

Fixes: 76443456227097179c1482 ("net: Move GSO csum into SKB_GSO_CB")
Reported-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_offload.c

@@ -49,6 +49,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 
 	/* setup inner skb. */
 	skb->encapsulation = 0;
+	SKB_GSO_CB(skb)->encap_level = 0;
 	__skb_pull(skb, tnl_hlen);
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, skb_inner_network_offset(skb));

net/ipv4/udp_offload.c

@@ -56,6 +56,7 @@ static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb,
 
 	/* setup inner skb. */
 	skb->encapsulation = 0;
+	SKB_GSO_CB(skb)->encap_level = 0;
 	__skb_pull(skb, tnl_hlen);
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, skb_inner_network_offset(skb));