verity: use <roothash>-verity as the device mapper name instead of libmnt_<image>

verity: use <roothash>-verity as the device mapper name instead of libmnt_<image>

If the roothash is the same among two volumes, then the images are guaranteed
to be equivalent. The filename of the image on the other hand does not imply
that the images are equivalent.
In systemd we open verity devices as '<roothash>-verity', so that we know for
sure we can reuse them. Do the same here, so that we are a bit more confident
that images can be reused before the safety check, and also so that we can
take advantage of the kernel refcounting together with images used by systemd,
as setting up verity is expensive.

Signed-off-by: Luca Boccassi <bluca@debian.org>