When the rightmost SACK block expands, rcv_lastsack should be updated.

(Fix for kern/78226).

Submitted by : Noritoshi Demizu <demizu at dd dot iij4u dot or dot jp>
Reviewed by  : Mohan Srinivasan (mohans at yahoo-inc dot com),
               Raja Mukerji (raja at moselle dot com).

sys/netinet/tcp_sack.c

@@ -463,6 +463,8 @@ tcp_sack_option(struct tcpcb *tp, struct tcphdr *th, u_char *cp, int optlen)
 			tp->snd_numholes++;
 			tcp_sack_globalholes++;
 		}
+		if (SEQ_LT(tp->rcv_lastsack, sack.end))
+			tp->rcv_lastsack = sack.end;
 	}
 	return (0);
 }