restore: No need to track pid of main shmems, open map_files/ via /proc/self

It basically reverts e189efc1
which was overdone one.

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