tcp: fix a size_t < 0 comparison in tcp_read_sock

<used> should be of type int (not size_t) since recv_actor can return
negative values and it is also used in a < 0 comparison.
Signed-off-by: NOctavian Purdila <opurdila@ixiacom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1209,7 +1209,8 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
 		return -ENOTCONN;
 	while ((skb = tcp_recv_skb(sk, seq, &offset)) != NULL) {
 		if (offset < skb->len) {
-			size_t used, len;
+			int used;
+			size_t len;
 
 			len = skb->len - offset;
 			/* Stop reading if we hit a patch of urgent data */