nexthop: Prepare new notification info

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: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>