mm: initialize PageEntry pe to NULL in PagemapEntry

Otherwise it contains trash and BUG at page-read.c:98 may be triggered.

https://bugzilla.openvz.org/show_bug.cgi?id=2633



Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>