zfs: list zvol: limit recursion depth to 1

zfs: list zvol: limit recursion depth to 1

To be correct in all cases, it's still necessary to filter by "pool"
in zfs_parse_zvol_list(), because $scfg->{pool} could be e.g.
'foo/vm-123-disk-0' which looks like an image name and would pass the
other "should skip"-checks in zfs_parse_zvol_list().

No change in the result of zfs_list_zvol() is intended.

Suggested-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>