m68k: mm: Improve kernel_page_table()

With the PTE-tables now only being 256 bytes, allocating a full page
for them is a giant waste. Start by improving the boot time allocator
such that init_mm initialization will at least have optimal memory
density.

Much thanks to Will Deacon in help with debugging and ferreting out
lost information on these dusty MMUs.

Notes:

 - _TABLE_MASK is reduced to account for the shorter (256 byte)
   alignment of pte-tables, per the manual, table entries should only
   ever have state in the low 4 bits (Used,WrProt,Desc1,Desc0) so it is
   still longer than strictly required. (Thanks Will!!!)

 - Also use kernel_page_table() for the 020/030 zero_pgtable case and
   consequently remove the zero_pgtable init hack (will fix up later).

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/20200131125403.768263973@infradead.org


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