Reset the last_sack_ack SACK hint for TCP input processing to ensure that the

hint is 0 when no SACK data is received to update the hint with. This was
accidentally omitted from r216753.

Sponsored by:	FreeBSD Foundation
MFC after:	10 weeks
X-MFC with:	216753

sys/netinet/tcp_input.c

@@ -1291,6 +1291,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 	short ostate = 0;
 #endif
 	thflags = th->th_flags;
+	tp->sackhint.last_sack_ack = 0;
 
 	/*
 	 * If this is either a state-changing packet or current state isn't