dm vdo wait-queue: rename to vdo_waitq_dequeue_waiter

Rename vdo_waitq_dequeue_next_waiter to vdo_waitq_dequeue_waiter.  The
"next" aspect of returned waiter is implied. "next" also isn't
informative ("oldest" would be). Removing "next_" adds symmetry to
vdo_waitq_enqueue_waiter().

Also fix whitespace and comments from previous waitq commit.

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