Prepare the new notification information so that it could be passed to
listeners in the new patch.

Changes since RFC:
* Add a blank line in __nh_notifier_single_info_init()
Signed-off-by: NIdo Schimmel <idosch@nvidia.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>