nexthop: fix uninitialized variable in nla_put_nh_group_stats()

The "*hw_stats_used" value needs to be set on the success paths to prevent
an uninitialized variable bug in the caller, nla_put_nh_group_stats().

Fixes: 5072ae00

 ("net: nexthop: Expose nexthop group HW stats to user space")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/f08ac289-d57f-4a1a-830f-cf9a0563cb9c@moroto.mountain


Signed-off-by: Jakub Kicinski <kuba@kernel.org>