libmount: ensure child hangs around until we persisted namespace

libmount: ensure child hangs around until we persisted namespace

When we create a new namespace in a child process to persist it we need
to ensure that the child hangs around. During exit the child will drop
all references to its namespaces and so by the time we call open we
might already fail to open the namespace. Fix this.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>