fix volume activation for ZFS subvols

fix volume activation for ZFS subvols

When using the path to request properties, and no ZFS file system is mounted
at that path, ZFS will fall back to the parent filesystem:

> # zfs unmount myzpool/subvol-172-disk-0
> # zfs get mounted /myzpool/subvol-172-disk-0
> NAME     PROPERTY  VALUE    SOURCE
> myzpool  mounted   yes      -
> # zfs get mounted myzpool/subvol-172-disk-0
> NAME                       PROPERTY  VALUE    SOURCE
> myzpool/subvol-172-disk-0  mounted   no       -

Thus, we cannot use the path and need to use the dataset directly.

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