fib: rules: remove duplicated nla policies

The attributes are identical in all implementations so move the ipv4 one
into the core and remove the per-family nla policies.

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