block: remove blk_flush_policy

Fold blk_flush_policy into the only caller to prepare for pending changes
to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20240617060532.127975-13-hch@lst.de


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