btrfs: have proper error handling in btrfs_init_reloc_root

create_reloc_root will return errors in the future, and __add_reloc_root
can return ENOMEM or EEXIST, so handle these errors properly.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -856,9 +856,15 @@ int btrfs_init_reloc_root(struct btrfs_trans_handle *trans,
 	reloc_root = create_reloc_root(trans, root, root->root_key.objectid);
 	if (clear_rsv)
 		trans->block_rsv = rsv;
+	if (IS_ERR(reloc_root))
+		return PTR_ERR(reloc_root);
 
 	ret = __add_reloc_root(reloc_root);
-	BUG_ON(ret < 0);
+	if (ret) {
+		/* Pairs with create_reloc_root */
+		btrfs_put_root(reloc_root);
+		return ret;
+	}
 	root->reloc_root = btrfs_grab_root(reloc_root);
 	return 0;
 }