Fix privileged create_netns function

In the patch [1] create_netns function from the Neutron's
privileged ip_lib module was changed in the way that it was
implemented in the same way like original pyroute2's create_netns
function.
Unfortunately during backporing that patch to stable/ussuri and
stable/train branches, we probably made mistake and instead of
calling pyroute2's private function _netns() we still called
public function netns() (which we tried to "reimplement" really).

So this patch updates it and changes to use private _netns() function
from pyroute2, in the same way like it is done in the branches newer
than stable/ussuri.

[1] https://review.opendev.org/c/openstack/neutron/+/778735

Related-Bug: #1917487
Change-Id: Id25ca8a67b52393a1c24c9d659189adc9c16a011

neutron/privileged/agent/linux/ip_lib.py

@@ -534,7 +534,7 @@ def create_netns(name, **kwargs):
     pid = os.fork()
     if pid == 0:
         try:
-            netns.create(name, libc=_get_cdll())
+            netns._create(name, libc=_get_cdll())
         except OSError as e:
             if e.errno != errno.EEXIST:
                 os._exit(1)