Properly follow linked namespace container for stats

Podman containers can specify that they get their network
namespace from another container. This is automatic in pods, but
any container can do it.

The problem is that these containers are not guaranteed to have a
network namespace of their own; it is perfectly valid to join the
network namespace of a --net=host container, and both containers
will end up in the host namespace. The code for obtaining network
stats did not account for this, and could cause segfaults as a
result. Fortunately, the fix is simple - the function we use to
get said stats already performs appropriate checks, so we just
need to recursively call it.

Fixes #5652

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/networking_linux.go

@@ -550,7 +550,7 @@ func getContainerNetNS(ctr *Container) (string, error) {
 		if err = c.syncContainer(); err != nil {
 			return "", err
 		}
-		return c.state.NetNS.Path(), nil
+		return getContainerNetNS(c)
 	}
 	return "", nil
 }