Btrfs: init relocate extent_io_tree with a mapping

Dave reported a NULL pointer deref.  This is caused because he thought he'd be
smart and add sanity checks to the extent_io bit operations, but he didn't
expect a tree to have a NULL mapping.  To fix this we just need to init the
relocation's processed_blocks with the btree_inode->i_mapping.  Thanks,

Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/relocation.c

@@ -4082,7 +4082,7 @@ out:
 	return inode;
 }
 
-static struct reloc_control *alloc_reloc_control(void)
+static struct reloc_control *alloc_reloc_control(struct btrfs_fs_info *fs_info)
 {
 	struct reloc_control *rc;
 
@@ -4093,7 +4093,8 @@ static struct reloc_control *alloc_reloc_control(void)
 	INIT_LIST_HEAD(&rc->reloc_roots);
 	backref_cache_init(&rc->backref_cache);
 	mapping_tree_init(&rc->reloc_root_tree);
-	extent_io_tree_init(&rc->processed_blocks, NULL);
+	extent_io_tree_init(&rc->processed_blocks,
+			    fs_info->btree_inode->i_mapping);
 	return rc;
 }
 
@@ -4110,7 +4111,7 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
 	int rw = 0;
 	int err = 0;
 
-	rc = alloc_reloc_control();
+	rc = alloc_reloc_control(fs_info);
 	if (!rc)
 		return -ENOMEM;
 
@@ -4311,7 +4312,7 @@ int btrfs_recover_relocation(struct btrfs_root *root)
 	if (list_empty(&reloc_roots))
 		goto out;
 
-	rc = alloc_reloc_control();
+	rc = alloc_reloc_control(root->fs_info);
 	if (!rc) {
 		err = -ENOMEM;
 		goto out;