dm vdo wait-queue: add proper namespace to interface

Rename various interfaces and structs associated with vdo's wait-queue,
e.g.: s/wait_queue/vdo_wait_queue/, s/waiter/vdo_waiter/, etc.

Now all function names start with "vdo_waitq_" or "vdo_waiter_".

Reviewed-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>