There is missing optimization in zero_p4d_populate() that can save some
memory when mapping zero shadow.  Implement it like as others.

Link: http://lkml.kernel.org/r/1494829255-23946-1-git-send-email-iamjoonsoo.kim@lge.comSigned-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>