Don't overpromote values when calculating len in tcp_output().

sbavail() returns u_int and sendwin is a uint32_t. Therefore, min() (which
operates on two u_int values) is able to correctly calculate the minimum
of these two arguments.

Reported by:	rrs
MFC after:	1 week
Sponsored by:	Netflix

sys/netinet/tcp_output.c

@@ -386,7 +386,7 @@ tcp_output(struct tcpcb *tp)
 	 */
 	if (sack_rxmit == 0) {
 		if (sack_bytes_rxmt == 0)
-			len = ((int32_t)ulmin(sbavail(&so->so_snd), sendwin) -
+			len = ((int32_t)min(sbavail(&so->so_snd), sendwin) -
 			    off);
 		else {
 			int32_t cwin;