Change vm_pageout_scan() to return a value indicating whether the free page
target was met.

Previously, vm_pageout_worker() itself checked the length of the free page
queues to determine whether vm_pageout_scan(pass >= 1)'s inactive queue scan
freed enough pages to meet the free page target.  Specifically,
vm_pageout_worker() used vm_paging_needed().  The trouble with
vm_paging_needed() is that it compares the length of the free page queues to
the wakeup threshold for the page daemon, which is much lower than the free
page target.  Consequently, vm_pageout_worker() could conclude that the
inactive queue scan succeeded in meeting its free page target when in fact
it did not; and rather than immediately triggering an all-out laundering
pass over the inactive queue, vm_pageout_worker() would go back to sleep
waiting for the free page count to fall below the page daemon wakeup
threshold again, at which point it will perform another limited (pass == 1)
scan over the inactive queue.

Changing vm_pageout_worker() to use vm_page_count_target() instead of
vm_paging_needed() won't work because any page allocations that happen
concurrently with the inactive queue scan will result in the free page count
being below the target at the end of a successful scan.  Instead, having
vm_pageout_scan() return a value indicating success or failure is the most
straightforward fix.