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>