netlink: Plumb "sender" through to ICMP generation

In order for a point-to-point netlink to correctly construct and
deliver ICMP, it needs to know which way the original packet was
going.  So provide the ICMP generation code with this information, in
the form of the "sender" client.

No functional change.

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