memcg: fix slab accounting in refill_obj_stock() trylock path

In the trylock path of refill_obj_stock(), mod_objcg_mlstate() should use
the real alloc/free bytes (i.e., nr_acct) for accounting, rather than
nr_bytes.

The user-visible impact is that the NR_SLAB_RECLAIMABLE_B and
NR_SLAB_UNRECLAIMABLE_B stats can end up being incorrect.

For example, if a user allocates a 6144-byte object, then before this
fix efill_obj_stock() calls mod_objcg_mlstate(..., nr_bytes=2048), even
though it should account for 6144 bytes (i.e., nr_acct).

When the user later frees the same object with kfree(),
refill_obj_stock() calls mod_objcg_mlstate(..., nr_bytes=6144). This
ends up adding 6144 to the stats, but it should be applying -6144
(i.e., nr_acct) since the object is being freed.

Link: https://lkml.kernel.org/r/20260226115145.62903-1-hao.li@linux.dev
Fixes: 200577f69f29 ("memcg: objcg stock trylock without irq disabling")
Signed-off-by: Hao Li <hao.li@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>