nexthop: Dispatch nexthop_select_path() by group type

The logic for selecting path depends on the next-hop group type. Adapt the
nexthop_select_path() to dispatch according to the group type.

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