shed_cake.py bug, unable to add a cake qdisc under a parent

[HeroesLament]

Hello team,

Due to the behavior of ipr.tc() popping the 'parent' key from kwarg and onto msg, this creates a situation where when fix_msg() in sched_cake.py runs, it overwrites the 'parent' key in msg with TC_H_ROOT's value, even if the parent had been properly set:

def fix_msg(msg, kwarg):
if 'parent' not in kwarg:
msg['parent'] = TC_H_ROOT

This prevents any cake qdisc from being applied as a child qdisc.