MFC r288025

MFC r288025
  Correct a non-fatal error in vm_pageout_worker().  vm_pageout_worker()
  should not assume that vm_pages_needed will remain set while it sleeps.
  Other threads can clear vm_pages_needed by performing a sufficient
  number of vm_page_free() calls, e.g., process termination.  The effect
  of this error was that vm_pageout_worker() would free and/or launder
  pages when, in fact, there was no shortage of free pages.

  Rewrite a nearby comment to describe all of the possible cases and not
  just the most common case.  The problem being that the comment made
  the most common case seem like the only case.

git-svn-id: svn://svn.freebsd.org/base/stable/10@288296 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f