sched_ext: Fix invalid kobj cast in scx_uevent()

sched_ext: Fix invalid kobj cast in scx_uevent()

When scx_alloc_and_add_sched() creates the sub-scheduler kset, it sets
sch->kobj as the parent. Because sch->kobj.kset points to scx_kset,
registering this sub-kset triggers a KOBJ_ADD uevent. The uevent walk
finds scx_kset and calls scx_uevent() with the sub-kset's kobject.

scx_uevent() unconditionally uses container_of() to cast the incoming
kobject to struct scx_sched, producing a wild pointer when the kobject
belongs to the kset itself rather than a scheduler instance. Accessing
sch->ops.name through this pointer causes a KASAN slab-out-of-bounds
read:

  BUG: KASAN: slab-out-of-bounds in string+0x3b6/0x4c0
  Read of size 1 at addr ffff888004d04348 by task scx_enable_help/748

  Call Trace:
   string+0x3b6/0x4c0
   vsnprintf+0x3ec/0x1550
   add_uevent_var+0x160/0x3a0
   scx_uevent+0x22/0x30
   kobject_uevent_env+0x5dc/0x1730
   kset_register+0x192/0x280
   scx_alloc_and_add_sched+0x130d/0x1c60
   ...

Fix this by checking the kobject's ktype against scx_ktype before
performing the cast, and returning 0 for non-matching kobjects.

Tested with vng and scx_qmap without triggering any KASAN errors.

Fixes: ebeca1f930ea ("sched_ext: Introduce cgroup sub-sched support")
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>