ovl: relax lookup error on mismatch origin ftype

ovl: relax lookup error on mismatch origin ftype

We get occasional reports of lookup errors due to mismatched
origin ftype from users that re-format a lower squashfs image.

Commit 13c6ad0f45fd ("ovl: document lower modification caveats")
tries to discourage the practice of re-formating lower layers and
describes the expected behavior as undefined.

Commit b0e0f69731cd ("ovl: restrict lower null uuid for "xino=auto"")
limits the configurations in which origin file handles are followed.

In addition to these measures, change the behavior in case of detecting
a mismatch origin ftype in lookup to issue a warning, not follow origin,
but not fail the lookup operation either.

That should make overall more users happy without any big consequences.

Link: https://lore.kernel.org/linux-unionfs/CAOQ4uxgPq9E9xxwU2CDyHy-_yCZZeymg+3n+-6AqkGGE1YtwvQ@mail.gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>