blk-mq: allow the hctx cpu hotplug notifier to return errors

Prepare this for the next patch which adds more smarts in the
plugging logic, so that we can save some memory.

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/blk-mq-cpu.c
Simple merge

diff --cc block/blk-mq.c
Simple merge

diff --cc block/blk-mq.h
Simple merge

diff --cc include/linux/blk-mq.h
Simple merge