Add some counters for per-VM page events.

For now, just count batched page queue state operations.
vm.stats.page.queue_ops counts the number of batch entries that
successfully completed, while queue_nops counts entries that had no
effect, which occurs when the queue operation had been completed before
the batch entry was processed.

Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: Intel, Netflix
Differential Revision: https://reviews.freebsd.org/D21782