ipv4: Cache input routes in fib_info nexthops.

Caching input routes is slightly simpler than output routes, since we
don't need to be concerned with nexthop exceptions.  (locally
destined, and routed packets, never trigger PMTU events or redirects
that will be processed by us).

However, we have to elide caching for the DIRECTSRC and non-zero itag
cases.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -82,6 +82,7 @@ struct fib_nh {
 	__be32			nh_saddr;
 	int			nh_saddr_genid;
 	struct rtable		*nh_rth_output;
+	struct rtable		*nh_rth_input;
 	struct fnhe_hash_bucket	*nh_exceptions;
 };