Starting with version 3.15, the kernel provides a mnt_id field in
/proc/<pid>/fdinfo/<fd>.  However, the value provided by the kernel for
AUFS file descriptors obtained by opening a file in /proc/<pid>/map_files
is incorrect.

Below is an example for a Docker container running Nginx.  The mntid
program below mimics CRIU by opening a file in /proc/1/map_files and
using the descriptor to obtain its mnt_id.  As shown below, mnt_id is
set to 22 by the kernel but it does not exist in the mount namespace of
the container.  Therefore, CRIU fails with the error:

	"Unable to look up the 22 mount"

In the global namespace, 22 is the root of AUFS (/var/lib/docker/aufs).

This patch sets the mnt_id of these AUFS descriptors to -1, mimicing
pre-3.15 kernel behavior.

	$ docker ps
	CONTAINER ID        IMAGE                    ...
	3850a63ee857        nginx-streaming:latest   ...
	$ docker exec -it 38 bash -i
	root@3850a63ee857:/# ps -e
	  PID TTY          TIME CMD
	    1 ?        00:00:00 nginx
	    7 ?        00:00:00 nginx
	   31 ?        00:00:00 bash
	   46 ?        00:00:00 ps
	root@3850a63ee857:/# ./mntid 1
	open("/proc/1/map_files/400000-4b8000") = 3
	cat /proc/49/fdinfo/3
	pos:	0
	flags:	0100000
	mnt_id:	22
	root@3850a63ee857:/# awk '{print $1 " " $2}' /proc/1/mountinfo
	87 58
	103 87
	104 87
	105 104
	106 104
	107 104
	108 87
	109 87
	110 87
	111 87
	root@3850a63ee857:/# exit
	$ grep 22 /proc/self/mountinfo
	22 21 8:1 /var/lib/docker/aufs /var/lib/docker/aufs ...
	44 22 0:35 / /var/lib/docker/aufs/mnt/<ID> ...
	$
Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>