overlay: create helper _overlay_scratch_mount_opts()

overlay: create helper _overlay_scratch_mount_opts()

The new overlayfs mount options lowerdir+,datadir+ don't fit well
into any of the existing _overlay_scratch_mount* helpers.
Add this new helper to reduce a common pattern of custom mount options.

Suggested-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>