It's possiable, if a parent task lives in a sub-mntns and
its child lives in the root mntns. A mount namespace is
restored before forking children, so all children are created
in parent's mount namespace. If a child lives in another
mount namespace, we need to move it into the required mntns.

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>