When we'll restore nested mount namespaces, all but root ones (sub-namespaces)
will be restored as sub-mounts in the root mount namespace. So mi->mountpoint
will be not '/' even if a mount is root for its mntns.

v2: s/is_root/is_ns_root/
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>