blk-mq: rename mq_flush_work struct request member

We will use this work_struct to requeue scsi commands from the
completion handler as well, so give it a more generic name.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

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

diff --cc include/linux/blkdev.h
Simple merge