Ok, I see.  At one point we had a special case to allow the overlay code
to write trusted.* xattrs for creating whiteouts.

However that is gone.  Therefore when overlayfs v1 (mount -t overlayfs)
is mounted, root in a user namespace also is not able to rm a file which
exists in the lower fs.

Some ways to fix this:

1. Add a special case in fs/xattr.c to allow the overlay code to create the 
trusted.overlay xattrs
2. In ovl_create_or_link(), target the override cred at init_user_ns.  Since we 
don't do that, the capabilities we are adding do not grant 
"capable(CAP_SYS_ADMIN)", only ns_capable.
3. Find another way to do this without requiring the trusted.overlay xattr.  It 
isn't needed for files so I don't know what the complications are, which 
require it to be done for directories.

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https://bugs.launchpad.net/bugs/1531747

Title:
  overlay: mkdir fails if directory exists in lowerdir in a user
  namespace

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