ipv4: rt_cache_valid must check expired routes

commit d2d68ba9fe8 (ipv4: Cache input routes in fib_info nexthops.)
introduced rt_cache_valid() helper. It unfortunately doesn't check if
route is expired before caching it.

I noticed sk_setup_caps() was constantly called on a tcp workload.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>