sched_ext: Fix build errors and unused label warning in non-cgroup configs

sched_ext: Fix build errors and unused label warning in non-cgroup configs

When building with SCHED_CLASS_EXT=y but CGROUPS=n, clang reports errors
for undeclared cgroup_put() and cgroup_get() calls, and a warning for the
unused err_stop_helper label.

EXT_SUB_SCHED is def_bool y depending only on SCHED_CLASS_EXT, but it
fundamentally requires cgroups (cgroup_path, cgroup_get, cgroup_put,
cgroup_id, etc.). Add the missing CGROUPS dependency to EXT_SUB_SCHED in
init/Kconfig.

Guard cgroup_put() and cgroup_get() in the common paths with:
  #if defined(CONFIG_EXT_GROUP_SCHED) || defined(CONFIG_EXT_SUB_SCHED)

Guard the err_stop_helper label with #ifdef CONFIG_EXT_SUB_SCHED since
all gotos targeting it are inside that same ifdef block.

Tested with both CGROUPS enabled and disabled.

Fixes: ebeca1f930ea ("sched_ext: Introduce cgroup sub-sched support")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603210903.IrKhPd6k-lkp@intel.com/
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>