tbf: stop wanton destruction of children (v2)

Several netem users use TBF for rate control. But every time the parameters
of TBF are changed it destroys the child qdisc, requiring reconfigation.
Better to just keep child qdisc and just notify it of changed limit.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>