ipv4: Constrain UFO fragment sizes to multiples of 8 bytes

Because the ip fragment offset field counts 8-byte chunks, ip
fragments other than the last must contain a multiple of 8 bytes of
payload.  ip_ufo_append_data wasn't respecting this constraint and,
depending on the MTU and ip option sizes, could create malformed
non-final fragments.

Google-Bug-Id: 5009328
Signed-off-by: Bill Sommerfeld <wsommerfeld@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -734,7 +734,7 @@ static inline int ip_ufo_append_data(struct sock *sk,
 			int getfrag(void *from, char *to, int offset, int len,
 			       int odd, struct sk_buff *skb),
 			void *from, int length, int hh_len, int fragheaderlen,
-			int transhdrlen, int mtu, unsigned int flags)
+			int transhdrlen, int maxfraglen, unsigned int flags)
 {
 	struct sk_buff *skb;
 	int err;
@@ -767,7 +767,7 @@ static inline int ip_ufo_append_data(struct sock *sk,
 		skb->csum = 0;
 
 		/* specify the length of each IP datagram fragment */
-		skb_shinfo(skb)->gso_size = mtu - fragheaderlen;
+		skb_shinfo(skb)->gso_size = maxfraglen - fragheaderlen;
 		skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
 		__skb_queue_tail(queue, skb);
 	}
@@ -831,7 +831,7 @@ static int __ip_append_data(struct sock *sk,
 	    (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len) {
 		err = ip_ufo_append_data(sk, queue, getfrag, from, length,
 					 hh_len, fragheaderlen, transhdrlen,
-					 mtu, flags);
+					 maxfraglen, flags);
 		if (err)
 			goto error;
 		return 0;