mnt: Carry run-time device ID in mount_info

When we're restoring fsnotify watchees we need to resolve
path to a handle at some mountpoint referred by @s_dev
member (device ID) which is saved inside image. This
ID actually may be changed at the every mount (say
one restores container after machine reboot) or in
case of container's migration.

Thus the test for overmounting in __open_mountpoint
will fail and we get an error.

Lets do a trick: introduce @s_dev_rt member which
is supposed to carry run-time device ID. When dumping
this member simply equal to traditional @s_dev fetched
from the procfs, but when restoring we fetch it from
stat call once mountpoint become alive.

https://jira.sw.ru/browse/PSBM-41610



v2:
 - predefine MOUNT_INVALID_DEV
 - use fetch_rt_stat instead of assigning device in restore_shared_options
 - copy @s_dev_rt in propagate_siblings and propagate_mount

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