dm vdo block-map: fix a few small nits

Rename 'pages' to 'num_pages' in distribute_page_over_waitq().
Update assert message in validate_completed_page() to model others.
Tweak line-wrapping on a comment that was needlessly long.

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