Refactor some page queue scanning code.

• Use a common subroutine to collect a batch of pages for processing.
• Use a common subroutine to detect pages that were logically dequeued after being added to a batch.
• Use batching in the laundry queue scan. Remove vm_pageout_fallback_object_lock() and vm_pageout_page_lock().
• Remove a racy assert from vm_page_alloc_check(). [1]

Reported by: pho [1]