ipv6: fix rt6_update_expires

Commit 1716a961 (ipv6: fix problem with expired dst cache) broke PMTU
discovery. rt6_update_expires() calls dst_set_expires(), which only updates
dst->expires if it has not been set previously (expires == 0) or if the new
expires is earlier than the current dst->expires.

rt6_update_expires() needs to zero rt->dst.expires, otherwise it will contain
ivalid data left over from rt->dst.from and will confuse dst_set_expires().
Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip6_fib.h

@@ -143,8 +143,14 @@ static inline void rt6_set_expires(struct rt6_info *rt, unsigned long expires)
 
 static inline void rt6_update_expires(struct rt6_info *rt, int timeout)
 {
-	if (!(rt->rt6i_flags & RTF_EXPIRES) && rt->dst.from)
+	if (!(rt->rt6i_flags & RTF_EXPIRES)) {
+		if (rt->dst.from)
 			dst_release(rt->dst.from);
+		/* dst_set_expires relies on expires == 0 
+		 * if it has not been set previously.
+		 */
+		rt->dst.expires = 0;
+	}
 
 	dst_set_expires(&rt->dst, timeout);
 	rt->rt6i_flags |= RTF_EXPIRES;