vm_page: use ref_count for NOFREE counter

In managing freed PG_NOFREE pages, use ref_count instead of pindex for
counting pages in freed PG_NOFREE blocks. No change in behavior is
expected. This restores the (useful) property that the pindex field is
of interest only for pages that are mapped to objects.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50112