[TUN]: Fix check for underflow

This check is wrong, gcc optimizes it away:

                if ((len -= sizeof(pi)) > len)
			return -EINVAL;

This could be responsible for the BUG. If len is 2 or 3 and TUN_NO_PI
isn't set it underflows. alloc_skb() allocates len + 2, which is 0 or
1 byte. skb_reserve tries to reserve 2 bytes and things explode in
skb_put.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -229,7 +229,7 @@ static __inline__ ssize_t tun_get_user(struct tun_struct *tun, struct iovec *iv,
 	size_t len = count;
 
 	if (!(tun->flags & TUN_NO_PI)) {
-		if ((len -= sizeof(pi)) > len)
+		if ((len -= sizeof(pi)) > count)
 			return -EINVAL;
 
 		if(memcpy_fromiovec((void *)&pi, iv, sizeof(pi)))