compel: use a correct name format for vma files in /proc/pid/map_files/

Currently we use the "map_files/%p-%p" format, but actually it should
be "map_files/%lx-%lx".

The kernel could handle both formats, but recently Alexey Dobriyan fixed
the kernel and it accept only the second format.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>