Fix two problems with the page daemon control loop.

Both issues caused the page daemon to erroneously go to sleep when
applications are consuming free pages at a high rate, leaving the
application threads blocked in VM_WAIT.

1. After completing an inactive queue scan, concurrent allocations may have prevented the page daemon from meeting the v_free_min threshold. In this case, the page daemon was going to sleep even when the inactive queue contained plenty of clean pages.
2. pagedaemon_wakeup() may be called without the free queues lock held. This can lead to a lost wakeup if a call occurs after the page daemon clears vm_pageout_wanted but before going to sleep.

Fix 1) by ensuring that we start a new inactive queue scan immediately
if v_free_count < v_free_min after a prior scan.

Fix 2) by adding a new subroutine, pagedaemon_wait(), called from
vm_wait() and vm_waitpfault(). It wakes up the page daemon if either
vm_pages_needed or vm_pageout_wanted is false, and atomically sleeps
on v_free_count.

Reported by: jeff
Reviewed by: alc
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D13424