rtnl: make ifla_policy static

The only place this is used outside rtnetlink.c is veth. So provide
wrapper function for this usage.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/veth.c
Simple merge

diff --cc include/net/rtnetlink.h
Simple merge

diff --cc net/core/rtnetlink.c
Simple merge