Need a tool to find possible dangling nodes

[SandyXSD]

What would you like to be added:
Add a way (maybe in the fsck command) to find dangling nodes in the FS.

Why is this needed:
A dangling node can not be accessed nor deleted, but it still takes space.

[SandyXSD]

One possible case we've seen is that a file is deleted after the corresponding trash sub-directory has been cleaned up, see #5414 .

[anysql]

Just for remind:

For MySQL meta, I take the following stop to clear the isolated files.

1, delete the jfs_edge where parent is a trash sub-directory but the parent inode does not exists.
2, delete the jfs_node where parent is a trash sub-directory but the parent inode does not exists.
3, delete the jfs_chunk where inode does not exist in jfs_node.
4, delete the jfs_chunk_ref where chunkid not in jfs_chunk.

Then run gc command to clear these files.