btrfs: make relocation use btrfs_read_tree_root()

Relocation has it's special roots, we don't want to save these in the
root cache either, so swap it to use btrfs_read_tree_root().  However
the reloc root does need REF_COWS set, so make sure we set it everywhere
we use this helper, as it no longer does the REF_COWS setting.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -1494,8 +1494,9 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
 	BUG_ON(ret);
 	kfree(root_item);
 
-	reloc_root = btrfs_read_fs_root(fs_info->tree_root, &root_key);
+	reloc_root = btrfs_read_tree_root(fs_info->tree_root, &root_key);
 	BUG_ON(IS_ERR(reloc_root));
+	set_bit(BTRFS_ROOT_REF_COWS, &reloc_root->state);
 	reloc_root->last_trans = trans->transid;
 	return reloc_root;
 }
@@ -4584,12 +4585,13 @@ int btrfs_recover_relocation(struct btrfs_root *root)
 		    key.type != BTRFS_ROOT_ITEM_KEY)
 			break;
 
-		reloc_root = btrfs_read_fs_root(root, &key);
+		reloc_root = btrfs_read_tree_root(root, &key);
 		if (IS_ERR(reloc_root)) {
 			err = PTR_ERR(reloc_root);
 			goto out;
 		}
 
+		set_bit(BTRFS_ROOT_REF_COWS, &reloc_root->state);
 		list_add(&reloc_root->root_list, &reloc_roots);
 
 		if (btrfs_root_refs(&reloc_root->root_item) > 0) {