ipv6 addrconf: Fix route lifetime setting in corner case.

Because of arithmetic overflow avoidance, the actual lifetime setting
(vs the value given by RA) did not increase monotonically around
0x7fffffff/HZ.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -475,7 +475,7 @@ int rt6_route_rcv(struct net_device *dev, u8 *opt, int len,
 	lifetime = ntohl(rinfo->lifetime);
 	if (lifetime == 0xffffffff) {
 		/* infinity */
-	} else if (lifetime > 0x7fffffff/HZ) {
+	} else if (lifetime > 0x7fffffff/HZ - 1) {
 		/* Avoid arithmetic overflow */
 		lifetime = 0x7fffffff/HZ - 1;
 	}