netlink: Break out netlink_client_deliver

netlink: Break out netlink_client_deliver

Provide a helper function for calling client->deliver, and use it at
the two call sites.  We are going to fix a bug in this area and want
to bring the implementations together so we have only one place to
fix.

A side effect is that packets via point-to-point links are properly
counted.

I have verified that I caught all of the call sites by experimentally
changing the field name from "deliver" to "deliverx" in netlink.h, and
inspecting the locations of the resulting compiler errors.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>