ovl: store and show the user provided lowerdir mount option

We are about to add new mount options for adding lowerdir one by one,
but those mount options will not support escaping.

For the existing case, where lowerdir mount option is provided as a colon
separated list, store the user provided (possibly escaped) string and
display it as is when showing the lowerdir mount option.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>