pagemap: Move PME_ constants to the header

This constants are system wide, so move them to mem.h
header for reuse sake.

[ xemul: It was kerndat.h in the patch ]

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>