vm_meter: Fix laundry accounting

Pages in PQ_UNSWAPPABLE should be considered part of the laundry.
Otherwise, on systems with no swap, the total amount of memory visible
to tools like top(1) decreases.

It doesn't seem very useful to have a dedicated counter for unswappable
pages, and updating applications accordingly would be painful, so just
lump them in with laundry for now.

PR: 280846
Reviewed by: bnovkov, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47216

(cherry picked from commit 6a07e67fb7a8b5687a492d9d70a10651d5933ff5)