mount: Verify for external shared mount

If we meet shared mount point without share master belonging
to us -- it means we might fail on restore, thus require both
master/slave mount peers to be collected on dump.

In other words, the output will be like

 | (00.077025) Error (mount.c:421): Mount 49 (master_id: 2 shared_id: 0) has unreachable sharing
 | (00.077123) Error (mount.c:472): Can't proceed 4237's mountinfo
 | (00.077865) Error (namespaces.c:442): Namespaces dumping finished with error 65280

https://bugzilla.openvz.org/show_bug.cgi?id=2608



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