ipv4: use separate genid for next hop exceptions

commit 13d82bf5 (ipv4: Fix flushing of cached routing informations)
added the support to flush learned pmtu information.

However, using rt_genid is quite heavy as it is bumped on route
add/change and multicast events amongst other places. These can
happen quite often, especially if using dynamic routing protocols.

While this is ok with routes (as they are just recreated locally),
the pmtu information is learned from remote systems and the icmp
notification can come with long delays. It is worthy to have separate
genid to avoid excessive pmtu resets.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -51,6 +51,7 @@ struct rtable;
 
 struct fib_nh_exception {
 	struct fib_nh_exception __rcu	*fnhe_next;
+	int				fnhe_genid;
 	__be32				fnhe_daddr;
 	u32				fnhe_pmtu;
 	__be32				fnhe_gw;