Refinements to r281079's sequential access optimization: Prefetched pages,

Refinements to r281079's sequential access optimization: Prefetched pages,
which constitute the majority of the pages that are processed by
vm_fault_dontneed(), are already near the tail of the inactive queue.  Only
the pages at faulting virtual addresses are actually moved by
vm_page_advise(..., MADV_DONTNEED).  However, vm_page_advise(...,
MADV_DONTNEED) is simultaneously too aggressive and passive for the moved
pages.  It makes most of these pages too easily reclaimable, and at the same
time it leaves enough pages in the active queue to trigger pageouts by the
page daemon.  Instead, with this change, the pages at faulting virtual
addresses are moved to the tail of the inactive queue, where they are
relatively close to the pages prefetched by the same page fault.

Discussed with:	jeff
Sponsored by:	EMC / Isilon Storage Division