fib: expand fib_rule_policy

Now that there is only one fib nla_policy there is no need to
keep the macro around.  Place it where its used.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>