ovl: Use str_on_off() helper in ovl_show_options()

Remove hard-coded strings by using the str_on_off() helper function.

Acked-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>