memcg: no stock lock for cpu hot-unplug

Previously on the cpu hot-unplug, the kernel would call drain_obj_stock()
with objcg local lock.  However local lock was not needed as the stock
which was accessed belongs to a dead cpu but we kept it there to disable
irqs as drain_obj_stock() may call mod_objcg_mlstate() which required irqs
disabled.  However there is no need to disable irqs now for
mod_objcg_mlstate(), so we can remove the local lock altogether from cpu
hot-unplug path.

Link: https://lkml.kernel.org/r/20250514184158.3471331-7-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: 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: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>