Only marker is guaranteed to be present on the queue after the relock
in vm_pageout_fallback_object_lock() and vm_pageout_page_lock(). The
check for the m->queue == queue assumes that the page does belong to a
queue.

Modify the 'unchanged' calculation bu dereferencing the marker tailq
pointers, which is known to belong to the queue. Since for a page m
linked to the queue, m->queue must be equal to the queue index, assert
this instead of checking.

In collaboration with: alc
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 2 weeks