phonet: Pass ifindex to fill_route().

We will convert route_doit() to RCU.

route_doit() will call rtm_phonet_notify() outside of RCU due
to GFP_KERNEL, so dev will not be available in fill_route().

Let's pass ifindex directly to fill_route().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>