Check TCP timestamp option flag so that the automatic receive buffer

scaling code does not use an uninitialized timestamp echo reply value
from the stack when timestamps are not enabled.

Differential Revision: https://reviews.freebsd.org/D3060
Reviewed by:	hiren
Approved by:	jmallett (mentor)
MFC after:	3 days
Sponsored by:	Norse Corp, Inc.

sys/netinet/tcp_input.c

@@ -1856,6 +1856,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 		 * the buffer to better manage the socket buffer resources.
 		 */
 			if (V_tcp_do_autorcvbuf &&
+			    (to.to_flags & TOF_TS) &&
 			    to.to_tsecr &&
 			    (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
 				if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&