In vm_object_backing_scan(), setting PG_REFERENCED on a page before

sleeping on that page is nonsensical.  Doing so reduces the likelihood
that the page daemon will reclaim the page before the thread waiting in
vm_object_backing_scan() is reawakened.  However, it does not guarantee
that the page is not reclaimed, so vm_object_backing_scan() restarts
after reawakening.  More importantly, this muddles the meaning of
PG_REFERENCED.  There is no reason to believe that the caller of
vm_object_backing_scan() is going to use (i.e., access) the contents of
the page.  There is especially no reason to believe that an access is
more likely because vm_object_backing_scan() had to sleep on the page.

Discussed with:	kib
MFC after:	3 weeks

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