ipv4: move fib4_has_custom_rules() helper to public header

So that we can use it in the next patch.
Additionally constify the helper argument.

Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>