net: Fix header size check for GSO case in recvmsg (af_packet)

Parameter 'len' is size_t type so it will never get negative.
Signed-off-by: Mariusz Kozlowski <mk@lab.zgora.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1610,9 +1610,11 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 
 		err = -EINVAL;
 		vnet_hdr_len = sizeof(vnet_hdr);
-		if ((len -= vnet_hdr_len) < 0)
+		if (len < vnet_hdr_len)
 			goto out_free;
 
+		len -= vnet_hdr_len;
+
 		if (skb_is_gso(skb)) {
 			struct skb_shared_info *sinfo = skb_shinfo(skb);