librbd: Skip tier pools in list_children and snap_unprotect

Fixes: #8187
Signed-off-by: Adam Crume <adamcrume@gmail.com>
Merge pull request #2619 from ceph/wip-8187

librbd: Skip tier pools in list_children and snap_unprotect

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>