This implements ptes-in-highmem for PPC, removes the quicklist
and zero-page stuff.  PTEs in highmem on SMP turned out to need
some significant changes to avoid deadlocks on the hash_table_lock
(now renamed to mmu_hash_lock).  The PMDs now contain the physical
address of the PTE page rather than the virtual address.
Anything that takes the mmu_hash_lock now operates with the DMMU
off to avoid MMU hash-table misses.