x86: fix bad pmd ffff810000207xxx(9090909090909090)

OGAWA Hirofumi and Fede have reported rare pmd_ERROR messages:
mm/memory.c:127: bad pmd ffff810000207xxx(9090909090909090).

Initialization's cleanup_highmap was leaving alignment filler
behind in the pmd for MODULES_VADDR: when vmalloc's guard page
would occupy a new page table, it's not allocated, and then
module unload's vfree hits the bad 9090 pmd entry left over.

Signed-off-by: Hugh Dickins <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/mm/init_64.c

···
 	pmd_t *last_pmd = pmd + PTRS_PER_PMD;
 
 	for (; pmd < last_pmd; pmd++, vaddr += PMD_SIZE) {
-		if (!pmd_present(*pmd))
+		if (pmd_none(*pmd))
 			continue;
 		if (vaddr < (unsigned long) _text || vaddr > end)
 			set_pmd(pmd, __pmd(0));