Fix the background laundering mechanism after r329882.

Rather than using the number of inactive queue scans as a metric for
how many clean pages are being freed by the page daemon, have the
page daemon keep a running counter of the number of pages it has freed,
and have the laundry thread use that when computing the background
laundering threshold.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D14884