dm vdo: remove remaining ring references

Lists are the new rings, so update all remaining references to rings to
talk about lists.

Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>