net: geneve: check skb is large enough for IPv4/IPv6 header

Check within geneve_xmit_skb/geneve6_xmit_skb that sk_buff structure
is large enough to include IPv4 or IPv6 header, and reject if not. The
geneve_xmit_skb portion and overall idea was contributed by Eric Dumazet.
Fixes a KMSAN-found uninit-value bug reported by syzbot at:
https://syzkaller.appspot.com/bug?id=abe95dc3e3e9667fc23b8d81f29ecad95c6f106f

Suggested-by: Eric Dumazet <[e-mail]>
Reported-by: [e-mail]
Signed-off-by: Phillip Potter <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/geneve.c

···
 	__be16 sport;
 	int err;
 
+	if (!pskb_network_may_pull(skb, sizeof(struct iphdr)))
+		return -EINVAL;
+
 	sport = udp_flow_src_port(geneve->net, skb, 1, USHRT_MAX, true);
 	rt = geneve_get_v4_rt(skb, dev, gs4, &fl4, info,
 			      geneve->cfg.info.key.tp_dst, sport);
···
 	__u8 prio, ttl;
 	__be16 sport;
 	int err;
+
+	if (!pskb_network_may_pull(skb, sizeof(struct ipv6hdr)))
+		return -EINVAL;
 
 	sport = udp_flow_src_port(geneve->net, skb, 1, USHRT_MAX, true);
 	dst = geneve_get_v6_dst(skb, dev, gs6, &fl6, info,