selftests/overlayfs: test specifying layers as O_PATH file descriptors

Verify that userspace can specify layers via O_PATH file descriptors.

Link: https://lore.kernel.org/r/20250210-work-overlayfs-v2-2-ed2a949b674b@kernel.org


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>