net/sched: sch_htb: limit htb_classify inner-class filter hops

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

net/sched: sch_htb: limit htb_classify inner-class filter hops

htb_classify() follows each filter-selected inner class by switching
to cl->filter_list, but never bounds the number of hops. A filter on
an inner class can point back to itself or to another inner class that
points back, creating an infinite loop in the packet classification
path with the qdisc lock held and BH disabled — a soft lockup / panic
from a single packet.

Bound the traversal with a hop counter and drop the packet with a
rate-limited warning once the bound is exceeded. The counter is
incremented at the point the inner filter chain is picked up, after the
TC_ACT_* switch has consumed the classifier verdict, so a terminal
TC_ACT_QUEUED/STOLEN/TRAP on the last permitted chain still sets *qerr
to __NET_XMIT_STOLEN and the packet is not charged as a drop by this
qdisc or its parent.

The bound is TC_HTB_MAXDEPTH, taken from HTB's own parameters rather than
from the qdisc hierarchy depth limit. Class levels run from 0 to
TC_HTB_MAXDEPTH - 1, so a traversal that strictly descends in level can
take at most TC_HTB_MAXDEPTH hops. That descent is what a sane
configuration does, but it is assumed here rather than enforced:
htb_find() resolves a classid against every class in the qdisc, so a
filter may equally select a sibling or an ancestor. The normal
root -> inner -> leaf path takes a single hop, so the bound does not
affect legitimate classification.

htb_classify() can now return NULL irrespective of CONFIG_NET_CLS_ACT,
whereas previously every NULL return sat inside that ifdef. The NULL
handler in htb_enqueue() therefore cannot stay conditional either, so
drop the ifdef around it. This matches hfsc_enqueue(), which has always
handled a NULL class unconditionally. Without it, a kernel built
without actions would dereference a NULL class instead of dropping.

Conditions to recreate the bug:
- CONFIG_NET_SCHED, CONFIG_NET_SCH_HTB, CONFIG_NET_CLS_U32,
CONFIG_LOCKUP_DETECTOR.
- Create an HTB qdisc on a device (e.g. lo), add an inner class
1:1 with a leaf child 1:10, install a root u32 filter selecting
1:1, and an inner-class u32 filter on 1:1 also selecting 1:1.
- Send one packet (ping). On the unfixed kernel the classify loop
spins with the qdisc lock held; with softlockup_panic=1 it panics.
- Reachable from unprivileged user via unshare -Urn (CAP_NET_ADMIN).