net: ip: make ip_route_input_rcu() return drop reasons

In this commit, we make ip_route_input_rcu() return drop reasons, which
come from ip_route_input_mc() and ip_route_input_slow().

The only caller of ip_route_input_rcu() is ip_route_input_noref(). We
adjust it by making it return -EINVAL on error and ignore the reasons that
ip_route_input_rcu() returns. In the following patch, we will make
ip_route_input_noref() returns the drop reasons.

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