As the next step in eliminating PG_CACHE pages, free rather than cache
pages in vm_pageout_scan(). The reactivation rate of cache pages created
by vm_pageout_scan() is extremely low; typically no more than 0.5% to
2.25% of the pages are ever reactivated. At the same time, caching pages
is more expensive than freeing them. For example, in a test with
PostgreSQL, this change reduced the amount of time spent in the inactive
queue scan by 1/6.

Differential Revision: https://reviews.freebsd.org/D2805
Reviewed by: kib
Sponsored by: EMC / Isilon Storage Division