zfs: use -r parameter when listing snapshots

zfs: use -r parameter when listing snapshots

Some versions of ZFS do not automatically display the child snapshots
when '-t snapshot' is used, but require '-r' to be present
additionally[1]. And in general, it's cleaner to specify the flag
explicitly.

Because of that, commit ac5c1af led to a regression[0] in the context
of ZFS over iSCSI with zfs_get_sorted_snapshot_list. Fix it, by adding
a -r flag again.

The volume_snapshot_info function is currently only used in the
context of replication and that requires a local ZFS pool, but it
would be affected by the same issue if it is ever used in the context
of ZFS over iSCSI, so also add -r there.

[0]: https://forum.proxmox.com/threads/102683/
[1]: https://forum.proxmox.com/threads/102683/post-442577

Fixes: 8c20d8a ("plugin: add volume_snapshot_info function")
Fixes: ac5c1af ("zfspool: add zfs_get_sorted_snapshot_list helper")
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>