Fix mounting ZFS snapshots whose dataset is not mounted below '/'

Fix mounting ZFS snapshots whose dataset is not mounted below '/'

Trying to back up a container with a ZFS dataset with non-standard mount
would fail, see [0].

This also removes the near-dead code
$name .= "\@$snapname";
when snapname is false-y, but defined and turns
the check for $snapname into one for definedness.

[0]: https://forum.proxmox.com/threads/lxc-backup-fails-unable-to-open-the-dataset-vzdump.64944/

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>