net: nexthop: Add hardware statistics notifications

Add hw_stats field to several notifier structures to communicate to the
drivers that HW statistics should be configured for nexthops within a given
group.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>