ipv4: Invalidate nexthop cache nh_saddr more correctly.

Any operation that:

1) Brings up an interface
2) Adds an IP address to an interface
3) Deletes an IP address from an interface

can potentially invalidate the nh_saddr value, requiring
it to be recomputed.

Perform the recomputation lazily using a generation ID.

Reported-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/ipv4.h

@@ -55,6 +55,7 @@ struct netns_ipv4 {
 	int current_rt_cache_rebuild_count;
 
 	atomic_t rt_genid;
+	atomic_t dev_addr_genid;
 
 #ifdef CONFIG_IP_MROUTE
 #ifndef CONFIG_IP_MROUTE_MULTIPLE_TABLES