btrfs: don't allocate a btrfs_bio for raid56 per-stripe bios

Except for the spurious initialization of ->device just after allocation
nothing uses the btrfs_bio, so just allocate a normal bio without extra
data.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -1082,11 +1082,8 @@ static int rbio_add_io_page(struct btrfs_raid_bio *rbio,
 	}
 
 	/* put a new bio on the list */
-	bio = btrfs_bio_alloc(bio_max_len >> PAGE_SHIFT ?: 1);
-	btrfs_bio(bio)->device = stripe->dev;
-	bio->bi_iter.bi_size = 0;
-	bio_set_dev(bio, stripe->dev->bdev);
-	bio->bi_opf = opf;
+	bio = bio_alloc(stripe->dev->bdev, max(bio_max_len >> PAGE_SHIFT, 1UL),
+			opf, GFP_NOFS);
 	bio->bi_iter.bi_sector = disk_start >> 9;
 	bio->bi_private = rbio;