ublk: remove UBLK_IO_F_PREFLUSH

REQ_PREFLUSH is turned into REQ_OP_FLUSH by the flush state machine
and thus never seen by a blk-mq based driver.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>