Btrfs: do not increment on bio_index one by one

'bio_index' is just a index, it's really not necessary to do increment
one by one.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/file-item.c

@@ -281,10 +281,10 @@ static int __btrfs_lookup_bio_sums(struct btrfs_root *root,
 found:
 		csum += count * csum_size;
 		nblocks -= count;
+		bio_index += count;
 		while (count--) {
 			disk_bytenr += bvec->bv_len;
 			offset += bvec->bv_len;
-			bio_index++;
 			bvec++;
 		}
 	}