Btrfs: re-add root to dead root list if we stop dropping it

If we stop dropping a root for whatever reason we need to add it back to the
dead root list so that we will re-start the dropping next transaction commit.
The other case this happens is if we recover a drop because we will add a root
without adding it to the fs radix tree, so we can leak it's root and commit root
extent buffer, adding this to the dead root list makes this cleanup happen.
Thanks,

Cc: stable@vger.kernel.org
Reported-by: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/extent-tree.c

@@ -7466,6 +7466,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 	int err = 0;
 	int ret;
 	int level;
+	bool root_dropped = false;
 
 	path = btrfs_alloc_path();
 	if (!path) {
@@ -7643,12 +7644,22 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 		free_extent_buffer(root->commit_root);
 		btrfs_put_fs_root(root);
 	}
+	root_dropped = true;
 out_end_trans:
 	btrfs_end_transaction_throttle(trans, tree_root);
 out_free:
 	kfree(wc);
 	btrfs_free_path(path);
 out:
+	/*
+	 * So if we need to stop dropping the snapshot for whatever reason we
+	 * need to make sure to add it back to the dead root list so that we
+	 * keep trying to do the work later.  This also cleans up roots if we
+	 * don't have it in the radix (like when we recover after a power fail
+	 * or unmount) so we don't leak memory.
+	 */
+	if (root_dropped == false)
+		btrfs_add_dead_root(root);
 	if (err)
 		btrfs_std_error(root->fs_info, err);
 	return err;