memcg: fix up drain_local_stock comment

Thomas and Vlastimil have noticed that the comment in drain_local_stock
doesn't quite make sense.  It talks about a synchronization with the
memory hotplug but there is no actual memory hotplug involvement here.  I
meant to talk about cpu hotplug here.  Fix that up and hopefuly make the
comment more helpful by referencing the cpu hotplug callback as well.

Link: https://lkml.kernel.org/r/YRDwOhVglJmY7ES5@dhcp22.suse.cz
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2178,8 +2178,9 @@ static void drain_local_stock(struct work_struct *dummy)
 	unsigned long flags;
 
 	/*
-	 * The only protection from memory hotplug vs. drain_stock races is
-	 * that we always operate on local CPU stock here with IRQ disabled
+	 * The only protection from cpu hotplug (memcg_hotplug_cpu_dead) vs.
+	 * drain_stock races is that we always operate on local CPU stock
+	 * here with IRQ disabled
 	 */
 	local_irq_save(flags);