mounts: mntns_collect_root in __open_mountpoint

We are going to support nested mount namespaces,
file descriptors can be from different namespaces,
so a namespace root must be updated for each of them.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>