x86: fix bad pmd ffff810000207xxx(9090909090909090)

x86: fix bad pmd ffff810000207xxx(9090909090909090)

upstream commit: 2884f110d5409714f3a04eeb6d2ecd77da66b242

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 <hugh@veritas.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Hugh notes:

 It's actually not a serious problem, but it does look as if it's a
 serious problem, so we should stamp it out.

Signed-off-by: Chris Wright <chrisw@sous-sol.org>