btrfs: use correct offset for reloc_inode in prealloc_file_extent_cluster()

In prealloc_file_extent_cluster(), btrfs_check_data_free_space() uses
wrong file offset for reloc_inode, it uses cluster->start and cluster->end,
which indeed are extent's bytenr. The correct value should be
cluster->[start|end] minus block group's start bytenr.

start bytenr   cluster->start
|              |     extent      |   extent   | ...| extent |
|----------------------------------------------------------------|
|                block group reloc_inode                         |

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/relocation.c

@@ -3038,12 +3038,14 @@ int prealloc_file_extent_cluster(struct inode *inode,
 	u64 num_bytes;
 	int nr = 0;
 	int ret = 0;
+	u64 prealloc_start = cluster->start - offset;
+	u64 prealloc_end = cluster->end - offset;
 
 	BUG_ON(cluster->start != cluster->boundary[0]);
 	inode_lock(inode);
 
-	ret = btrfs_check_data_free_space(inode, cluster->start,
+	ret = btrfs_check_data_free_space(inode, prealloc_start,
-					  cluster->end + 1 - cluster->start);
+					  prealloc_end + 1 - prealloc_start);
 	if (ret)
 		goto out;
 
@@ -3064,8 +3066,8 @@ int prealloc_file_extent_cluster(struct inode *inode,
 			break;
 		nr++;
 	}
-	btrfs_free_reserved_data_space(inode, cluster->start,
+	btrfs_free_reserved_data_space(inode, prealloc_start,
-				       cluster->end + 1 - cluster->start);
+				       prealloc_end + 1 - prealloc_start);
 out:
 	inode_unlock(inode);
 	return ret;