net/sched: Discuss about adding a new kernel parameter to set the default value of flow_limit

[r30009329]

How about adding a new kernel parameter to set the default value of flow_limit
 when the default qidsc is set to fq? Although We can use the tc to modify the
 default value of flow_limit, it is more convenient to use a kernel parameter to
 set the default value, especially in scenarios where the tc command is
 inconvenient or cannot be used.