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>

arch/m68k/mm/init.c

@@ -42,7 +42,6 @@ EXPORT_SYMBOL(empty_zero_page);
 
 #if !defined(CONFIG_SUN3) && !defined(CONFIG_COLDFIRE)
 extern void init_pointer_table(unsigned long ptable);
-extern pmd_t *zero_pgtable;
 #endif
 
 #ifdef CONFIG_MMU
@@ -135,10 +134,6 @@ static inline void init_pointer_tables(void)
 		if (pud_present(*pud))
 			init_pointer_table(pgd_page_vaddr(kernel_pg_dir[i]));
 	}
-
-	/* insert also pointer table that we used to unmap the zero page */
-	if (zero_pgtable)
-		init_pointer_table((unsigned long)zero_pgtable);
 #endif
 }