ovl: relax requirement for uuid=off,index=on

ovl: relax requirement for uuid=off,index=on

uuid=off,index=on required that all upper/lower directories are on the
same filesystem.

Relax the requirement so that only all the lower directories need to be
on the same filesystem.

Reported-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20260114-tonyk-get_disk_uuid-v1-3-e6a319e25d57@igalia.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>