m68knommu: set ZERO_PAGE() to the allocated zeroed page

The non-MMU m68k pagetable ZERO_PAGE() macro is being set to the
somewhat non-sensical value of "virt_to_page(0)". The zeroth page
is not in any way guaranteed to be a page full of "0". So the result
is that ZERO_PAGE() will almost certainly contain random values.

We already allocate a real "empty_zero_page" in the mm setup code shared
between MMU m68k and non-MMU m68k. It is just not hooked up to the
ZERO_PAGE() macro for the non-MMU m68k case.

Fix ZERO_PAGE() to use the allocated "empty_zero_page" pointer.

I am not aware of any specific issues caused by the old code.

Link: https://lore.kernel.org/linux-m68k/[e-mail]/T/#t
Reported-by: Hugh Dickens <[e-mail]>
Signed-off-by: Greg Ungerer <[e-mail]>

arch/m68k/include/asm/pgtable_no.h

···
  * ZERO_PAGE is a global shared page that is always zero: used
  * for zero-mapped memory areas etc..
  */
-#define ZERO_PAGE(vaddr)	(virt_to_page(0))
+extern void *empty_zero_page;
+#define ZERO_PAGE(vaddr)	(virt_to_page(empty_zero_page))
 
 /*
  * All 32bit addresses are effectively valid for vmalloc...