ip: limit use of gso_size to udp

The ipcm(6)_cookie field gso_size is set only in the udp path. The ip
layer copies this to cork only if sk_type is SOCK_DGRAM. This check
proved too permissive. Ping and l2tp sockets have the same type.

Limit to sockets of type SOCK_DGRAM and protocol IPPROTO_UDP to
exclude ping sockets.

v1 -> v2
- remove irrelevant whitespace changes

Fixes: bec1f6f697 ("udp: generate gso with UDP_SEGMENT")
Reported-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1219,7 +1219,8 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
 	if (mtu < IPV6_MIN_MTU)
 		return -EINVAL;
 	cork->base.fragsize = mtu;
-	cork->base.gso_size = sk->sk_type == SOCK_DGRAM ? ipc6->gso_size : 0;
+	cork->base.gso_size = sk->sk_type == SOCK_DGRAM &&
+			      sk->sk_protocol == IPPROTO_UDP ? ipc6->gso_size : 0;
 
 	if (dst_allfrag(xfrm_dst_path(&rt->dst)))
 		cork->base.flags |= IPCORK_ALLFRAG;