[SRU Xenial] LP#1618572 -- overlayfs whiteout rename stable regression

[Andy Whitcroft]

We have a regression in overlayfs when we attempt to rename a file over
a whiteout in some circumstances.  This is limited to overlayfs legacy
whiteout mode and to a subset of upper layer filesystems (including ramfs).
This was detected by a failure in cloud-init in overlayroot systems.

This was introduced in a stable update which had to be ported relative
to the Ubuntu delta.  The issue is in 4.4.0-35.54 which is already in
xenial-updates.

Tested using the reproducer script from Stefan and also by Stefan in the
overlayroot environment as per the original bug reporter.

Proposing for SRU to xenial.

-apw