btrfs: use bio_kmalloc in __alloc_device

Use bio_kmalloc instead of open coding it.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Acked-by: NDamien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/btrfs/volumes.c

@@ -421,7 +421,7 @@ static struct btrfs_device *__alloc_device(struct btrfs_fs_info *fs_info)
 	 * Preallocate a bio that's always going to be used for flushing device
 	 * barriers and matches the device lifespan
 	 */
-	dev->flush_bio = bio_alloc_bioset(GFP_KERNEL, 0, NULL);
+	dev->flush_bio = bio_kmalloc(GFP_KERNEL, 0);
 	if (!dev->flush_bio) {
 		kfree(dev);
 		return ERR_PTR(-ENOMEM);