dm vdo block-map: optimize enter_zone_read_only_mode

Rather than incrementally dequeue from the zone->flush_waiters
vdo_wait_queue, simply re-initialize it.

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