powerpc/pmap: Fix wired memory leak in booke64 page directories

Properly handle reference counts in the 64-bit pmap page directories.
Otherwise all page table pages would leak due to over-referencing. This
would cause a quick enter to swap on a desktop system (AmigaOne X5000) when
quitting and rerunning applications, or just building world.

Add an INVARIANTS check to validate no leakage at pmap release time.