sched_ext, sched/core: Fix build failure when !FAIR_GROUP_SCHED && EXT_GROUP_SCHED

While collecting SCX related fields in struct task_group into struct
scx_task_group, 6e6558a ("sched_ext, sched/core: Factor out struct
scx_task_group") forgot update tg->scx_weight usage in tg_weight(), which
leads to build failure when CONFIG_FAIR_GROUP_SCHED is disabled but
CONFIG_EXT_GROUP_SCHED is enabled. Fix it.

Fixes: 6e6558a ("sched_ext, sched/core: Factor out struct scx_task_group")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509170230.MwZsJSWa-lkp@intel.com/
Tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

kernel/sched/core.c

@@ -9551,7 +9551,7 @@ static unsigned long tg_weight(struct task_group *tg)
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	return scale_load_down(tg->shares);
 #else
-	return sched_weight_from_cgroup(tg->scx_weight);
+	return sched_weight_from_cgroup(tg->scx.weight);
 #endif
 }