Reduce the size of pv entries by 15%. This saves 1MB of KVA for mapping

pv entries per 1GB of user virtual memory.  (eg: if we had 1GB file was
mmaped into 30 processes, that would theoretically reduce the KVA demand by
30MB for pv entries.  In reality though, we limit pv entries so we don't
have that many at once.)

We used to store the vm_page_t for the page table page.  But we recently
had the pa of the ptp, or can calculate it fairly quickly.  If we wanted
to avoid the shift/mask operation in pmap_pde(), we could recover the
pa but that means we have to store it for a while.

This does not measurably change performance.

Suggested by:  alc
Tested by:  alc

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