The fsmagic check was always performed on "data-root" (`/var/lib/docker`),
not on the storage-driver's home directory (e.g. `/var/lib/docker/<somedriver>`).
This caused detection to be done on the wrong filesystem in situations
where `/var/lib/docker/<somedriver>` was a mount, and a different
filesystem than `/var/lib/docker` itself.
This patch checks if the storage-driver's home directory exists, and only
falls back to `/var/lib/docker` if it doesn't exist.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
