sch_htb: fix refcount leak in htb_parent_to_leaf_offload

In the Linux kernel, the following vulnerability has been resolved:

sch_htb: fix refcount leak in htb_parent_to_leaf_offload

The commit ae81feb7338c ("sch_htb: fix null pointer dereference

on a null new_q") fixes a NULL pointer dereference bug, but it

is not correct.

Because htb_graft_helper properly handles the case when new_q

is NULL, and after the previous patch by skipping this call

which creates an inconsistency : dev_queue->qdisc will still

point to the old qdisc, but cl->parent->leaf.q will point to

the new one (which will be noop_qdisc, because new_q was NULL).

The code is based on an assumption that these two pointers are

the same, so it can lead to refcount leaks.

The correct fix is to add a NULL pointer check to protect

qdisc_refcount_inc inside htb_parent_to_leaf_offload.