block: permit PREFLUSH and POSTFLUSH without prepare_flush_fn

This is preparation for removing q->prepare_flush_fn.

Temporarily, blk_queue_ordered() permits QUEUE_ORDERED_DO_PREFLUSH and
QUEUE_ORDERED_DO_POSTFLUSH without prepare_flush_fn.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>