mm/shrinker: fix bogus set_shrinker_bit() with cgroup.memory=nokmem

With cgroup.memory=nokmem, shrinker_memcg_alloc() bails out early and
never allocates an id, so shrinker->id keeps the 0 it got from the
kzalloc() in shrinker_alloc().  __list_lru_init() then copies that 0 into
lru->shrinker_id, where it looks like a valid bit index.

Nothing calls expand_shrinker_info() on nokmem either, so shrinker_nr_max
stays 0 and every memcg ends up with an empty map (map_nr_max == 0).

deferred_split_folio() hands a real memcg to __list_lru_add() regardless
of whether the lru is memcg aware, so the first THP queued in a cgroup
does set_shrinker_bit(memcg, nid, 0) and trips the bounds check:

WARNING: mm/shrinker.c:212 at set_shrinker_bit+0x7d/0x90, CPU#126
Call Trace:
 <TASK>
 deferred_split_folio+0x18c/0x220
 map_anon_folio_pmd_nopf+0xdd/0x130
 map_anon_folio_pmd_pf+0x14/0xb0
 do_huge_pmd_anonymous_page+0x1a1/0x620
 __handle_mm_fault+0xea9/0x10d0
 handle_mm_fault+0xe5/0x320
 do_user_addr_fault+0x1cc/0x870
 exc_page_fault+0x81/0x1b0
 asm_exc_page_fault+0x27/0x30
 </TASK>

Harmless, the WARN_ON_ONCE() is what keeps the out of bounds unit[] read
from happening, but the id should not look valid in the first place. 
Clear it before returning.

Two other spots could paper over this: drop the id in __list_lru_init()
when nokmem turns memcg_aware off, or make deferred_split_folio() pass
NULL like list_lru_add_obj() does.  Both leave shrinker->id lying around
for the next caller, so fix it where the id is handed out.

Link: https://lore.kernel.org/20260902073800.305481-1-jiayuan.chen@linux.dev
Fixes: fafaeceb89 ("mm: switch deferred split shrinker to list_lru")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shrinker.c

@@ -227,6 +227,8 @@ static int shrinker_memcg_alloc(struct shrinker *shrinker)
 {
 	int id;
 
+	shrinker->id = -1;
+
 	if (mem_cgroup_disabled())
 		return -ENOSYS;
 	if (mem_cgroup_kmem_disabled() && !(shrinker->flags & SHRINKER_NONSLAB))