net: ip: make ip_route_input_slow() return drop reasons

In this commit, we make ip_route_input_slow() return skb drop reasons,
and following new skb drop reasons are added:

  SKB_DROP_REASON_IP_INVALID_DEST

The only caller of ip_route_input_slow() is ip_route_input_rcu(), and we
adjust it by making it return -EINVAL on error.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>