recovery: use ensure_volume_unmounted in format_volume

 * Using ensure_path_unmounted on "/" when attempting to format
   system on P SAR can fail with -EINVAL.
   As we already introduced the ensure_volume_unmounted function
   to handle unmounting system when it's mounted at /mnt/system,
   use it in format_volume too.

Change-Id: If1ab12e64c919ac6785fad406390a9001768007c