tun: Fixed unsigned/signed comparison

Validated that this was actually using the unsigned comparison with gdb.
Signed-off-by: NAlex Gartrell <agartrell@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1501,7 +1501,7 @@ static int tun_recvmsg(struct kiocb *iocb, struct socket *sock,
 		goto out;
 	}
 	ret = tun_do_read(tun, tfile, &m->msg_iter, flags & MSG_DONTWAIT);
-	if (ret > total_len) {
+	if (ret > (ssize_t)total_len) {
 		m->msg_flags |= MSG_TRUNC;
 		ret = flags & MSG_TRUNC ? ret : total_len;
 	}