genetlink: add and use genl_set_err()

Add a static inline to generic netlink to wrap netlink_set_err()
to make it easier to use here - use it in openvswitch (the only
generic netlink user of netlink_set_err()).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

diff --cc net/openvswitch/datapath.c
Simple merge

diff --cc net/openvswitch/dp_notify.c
Simple merge