Run dirty pages through the inactive queue before the laundry queue.

This change modifies the active queue aging scan to place dirty pages in
the inactive queue rather than the laundry queue. vm_page_advise() is
modified similarly.

One side effect of this change is that dirty pages are given more time
to be reactivated instead of being paged out. However, this is not its
primary motivation. Pushing dirty pages through the inactive queue
establishes a temporal relationship between the inactive and laundry
queues and results in a more cohesive LRU page replacement mechanism. In
the absence of this change, the pagedaemon and laundry threads have
little information about each other's activity, and we can say little
about the relative ages of the pages at the beginning of the inactive
and laundry queues at a given point in time. However, in general we
would prefer to avoid laundering a given page until some number of clean
pages have been reclaimed in an attempt to satisfy a page shortage. The
implemented laundering policy uses the ratio of dirty to clean inactive
pages to set a minimum threshold for laundering, but without a shared
queue, we cannot be satisfied that all less-recently-used clean pages
have been reclaimed before deciding to launder a dirty page.

A downside to this change is that the pagedaemons are forced to expend
more CPU cycles handling dirty pages, as they must now examine dirty
pages twice. However, this cost is small relative to that of a swap
pageout. It may be reasonable to move vnode-backed pages directly to the
laundry queue, but this change aims to minimize differences in behaviour
with respect to HEAD.

Reviewed by: alc