m68k: mm: Fully initialize the page-table allocator

Also iterate the PMD tables to populate the PTE table allocator. This
also fully replaces the previous zero_pgtable hack.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200131125403.938797587@infradead.org


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>