hide overlapping paths

Hello!

Sometimes it easier to put all overlayfs into one folder. But it raising overlapping issue which causing fuser-overlayfs to stuck.

For example we have following folders layout:

* lower1
* lower2
* file1
* file2
* lower3
* file3
* lower4/data
* overlay

First we mount:

lower2+lower3 -> lower4/data

(as result all lower2 and lower3 would be visible in lower4/data: file1, file2, file3)

Then we mount:

lower1 + lower4 -> lower1/overlay

Now if we call 'find lower1' it will stuck because it trying to read 'lower1/overlay/overlay' directory.

Would be nice to keep overlapping feature but prevent stucking by hiding all overlapping directories from view:

* lower1/overlay/lower2
* lower1/overlay/lower3
* lower1/overlay/lower4
* lower1/overlay/overlay

Start by reproducing the nested lower2+lower3 to lower4/data and lower1+lower4 to lower1/overlay mounts, then run find on lower1. Inspect the overlay path handling involved in exposing nested directories. Done means find lower1 no longer hangs and the overlapping lower2, lower3, lower4, and overlay directories are hidden from view.

Written by the indexing model from the issue text.