block: move flush initialization to blk_flush_init

These fields are always used with the flush request, so
initialize them together.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

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

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