block: use REQ_FLUSH in blkdev_issue_flush()

Update blkdev_issue_flush() to use new REQ_FLUSH interface.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>