[UDP]: Fix length check.

Rémi Denis-Courmont wrote:
> Right. By the way, shouldn't "len" rather be signed in there?
> 
> 		unsigned int len;
> 
> 		/* if we're overly short, let UDP handle it */
> 		len = skb->len - sizeof(struct udphdr);
> 		if (len <= 0)
> 			goto udp;

It should, but the < 0 case can't happen since __udp4_lib_rcv
already makes sure that we have at least a complete UDP header.

Anyways, this patch fixes it.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>