ipv4: Move fib_has_custom_local_routes outside of IP_MULTIPLE_TABLES.

> net/ipv4/fib_frontend.c: In function 'fib_validate_source':
> net/ipv4/fib_frontend.c:411:16: error: 'struct netns_ipv4' has no member named 'fib_has_custom_local_routes'
>    if (net->ipv4.fib_has_custom_local_routes)
>                 ^
> net/ipv4/fib_frontend.c: In function 'inet_rtm_newroute':
> net/ipv4/fib_frontend.c:773:12: error: 'struct netns_ipv4' has no member named 'fib_has_custom_local_routes'
>    net->ipv4.fib_has_custom_local_routes = true;
>             ^

Fixes: 6e617de8 ("net: avoid a full fib lookup when rp_filter is disabled.")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>