Btrfs: don't allow a subvol to be deleted if it is the default subovl

Eric pointed out that btrfs will happily allow you to delete the default subvol.
This is a problem obviously since the next time you go to mount the file system
it will freak out because it can't find the root.  Fix this by adding a check to
see if our default subvol points to the subvol we are trying to delete, and if
it does not allowing it to happen.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/ioctl.c

@@ -1726,13 +1726,28 @@ out:
 static noinline int may_destroy_subvol(struct btrfs_root *root)
 {
 	struct btrfs_path *path;
+	struct btrfs_dir_item *di;
 	struct btrfs_key key;
+	u64 dir_id;
 	int ret;
 
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;
 
+	/* Make sure this root isn't set as the default subvol */
+	dir_id = btrfs_super_root_dir(root->fs_info->super_copy);
+	di = btrfs_lookup_dir_item(NULL, root->fs_info->tree_root, path,
+				   dir_id, "default", 7, 0);
+	if (di && !IS_ERR(di)) {
+		btrfs_dir_item_key_to_cpu(path->nodes[0], di, &key);
+		if (key.objectid == root->root_key.objectid) {
+			ret = -ENOTEMPTY;
+			goto out;
+		}
+		btrfs_release_path(path);
+	}
+
 	key.objectid = root->root_key.objectid;
 	key.type = BTRFS_ROOT_REF_KEY;
 	key.offset = (u64)-1;