dm-thin: use blkdev_issue_flush instead of open coding it

Use blkdev_issue_flush, which uses an on-stack bio instead of an
opencoded version with a bio embedded into struct pool.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220124091107.642561-9-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-thin.c

@@ -282,8 +282,6 @@ struct pool {
 	struct dm_bio_prison_cell **cell_sort_array;
 
 	mempool_t mapping_pool;
-
-	struct bio flush_bio;
 };
 
 static void metadata_operation_failed(struct pool *pool, const char *op, int r);
@@ -2906,7 +2904,6 @@ static void __pool_destroy(struct pool *pool)
 	if (pool->next_mapping)
 		mempool_free(pool->next_mapping, &pool->mapping_pool);
 	mempool_exit(&pool->mapping_pool);
-	bio_uninit(&pool->flush_bio);
 	dm_deferred_set_destroy(pool->shared_read_ds);
 	dm_deferred_set_destroy(pool->all_io_ds);
 	kfree(pool);
@@ -2987,7 +2984,6 @@ static struct pool *pool_create(struct mapped_device *pool_md,
 	pool->low_water_triggered = false;
 	pool->suspended = true;
 	pool->out_of_data_space = false;
-	bio_init(&pool->flush_bio, NULL, 0);
 
 	pool->shared_read_ds = dm_deferred_set_create();
 	if (!pool->shared_read_ds) {
@@ -3194,13 +3190,8 @@ static void metadata_low_callback(void *context)
 static int metadata_pre_commit_callback(void *context)
 {
 	struct pool *pool = context;
-	struct bio *flush_bio = &pool->flush_bio;
-
-	bio_reset(flush_bio);
-	bio_set_dev(flush_bio, pool->data_dev);
-	flush_bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
 
-	return submit_bio_wait(flush_bio);
+	return blkdev_issue_flush(pool->data_dev);
 }
 
 static sector_t get_dev_size(struct block_device *bdev)