sch_hfsc: make hfsc_qlen_notify() idempotent

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: sch_hfsc: make hfsc_qlen_notify() idempotent hfsc_qlen_notify() is not idempotent either and not friendly to its callers, like fq_codel_dequeue(). Let's make it idempotent to ease qdisc_tree_reduce_backlog() callers' life: 1. update_vf() decreases cl->cl_nactive, so we can check whether it is non-zero before calling it. 2. eltree_remove() always removes RB node cl->el_node, but we can use RB_EMPTY_NODE() + RB_CLEAR_NODE() to make it safe.

References

• https://git.kernel.org/stable/c/9a5fd5c2f4d4afdd5e405083ee53e0789ce76956
• https://git.kernel.org/stable/c/72c61ffbeeb8c50f6d4d70c65d3283aa1bac57a7
• https://git.kernel.org/stable/c/a5efc95a33bd4fcb879250852828cc58c7862970
• https://git.kernel.org/stable/c/0475c85426b18eccdcb7f9fb58d8f8e9c6c58c87
• https://git.kernel.org/stable/c/9030a91235ae4845ec71902c3e0cecfc9ed1f2df
• https://git.kernel.org/stable/c/d06476714d2819b550e0cc39222347e2c8941c9d
• https://git.kernel.org/stable/c/c1175c4ad01dbc9c979d099861fa90a754f72059
• https://git.kernel.org/stable/c/51eb3b65544c9efd6a1026889ee5fb5aa62da3bb