sched: add missing group change to qfq_change_class

[Resending again, as the text was corrupted by the email client]

To speed up operations, QFQ internally divides classes into
groups. Which group a class belongs to depends on the ratio between
the maximum packet length and the weight of the class. Unfortunately
the function qfq_change_class lacks the steps for changing the group
of a class when the ratio max_pkt_len/weight of the class changes.

For example, when the last of the following three commands is
executed, the group of class 1:1 is not correctly changed:

tc disc add dev XXX root handle 1: qfq
tc class add dev XXX parent 1: qfq classid 1:1 weight 1
tc class change dev XXX parent 1: classid 1:1 qfq weight 4

Not changing the group of a class does not affect the long-term
bandwidth guaranteed to the class, as the latter is independent of the
maximum packet length, and correctly changes (only) if the weight of
the class changes. In contrast, if the group of the class is not
updated, the class is still guaranteed the short-term bandwidth and
packet delay related to its old group, instead of the guarantees that
it should receive according to its new weight and/or maximum packet
length. This may also break service guarantees for other classes.
This patch adds the missing operations.

Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: David S. Miller <davem@davemloft.net>