add workaround for zfs rollback bug

add workaround for zfs rollback bug

as described in the zfs bug https://github.com/openzfs/zfs/issues/10931
the kernel keeps around cached data from mmaps after a rollback, thus
having invalid data in files that were allegedly rolled back

to workaround this (until a real fix comes along), we unmount the subvol,
invalidating the kernel cache anyway

the dataset gets mounted on the next 'activate_volume' again

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>