net/sched: bound qdisc_pkt_len to prevent qdisc soft lockup

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

net/sched: bound qdisc_pkt_len to prevent qdisc soft lockup

qdisc_get_stab() accepts a user-supplied size table, and
__qdisc_calculate_pkt_len() amplifies qdisc_pkt_len() through the
overhead, the size-table data (u16), and size_log (up to
STAB_SIZE_LOG_MAX). A crafted stab can therefore set qdisc_pkt_len()
to ~1 GiB for an ordinary skb. Per-flow deficit schedulers such as
DRR and ETS replenish one quantum per loop iteration; with a tiny
quantum (1) they spin billions of times under the qdisc lock,
producing a soft lockup / RCU stall as illustrated by vega@nebusec.ai.

Cap the final qdisc_pkt_len() to QDISC_PKT_LEN_MAX so the size-table
amplification cannot drive deficit schedulers into an unbounded loop.
A legitimate size table (e.g. qfq's overhead 999999999, which is
handled by dropping) is still accepted.

Introduce cap QDISC_PKT_LEN_MAX (1 << 20) = 1 MiB which is well above
any legitimate single-skb wire length: the largest current skb->len
is GSO_MAX_SIZE (524280), and an ATM-style size table (53/48 cell tax)
amplifies that to ~578 KB, both comfortably below 1 MiB. At the same
time, 1 MiB bounds the deficit refill loop to ~1M iterations per
packet with quantum=1, which completes in a few milliseconds well
under the demonstrated softlockup threshold (~10^9 iterations).

Conditions to recreate the bug:
- CONFIG_NET_SCHED=y, CONFIG_NET_SCH_DRR=y (or CONFIG_NET_SCH_ETS=y).
- Attach a DRR (or ETS) root qdisc with a crafted TCA_STAB that
amplifies qdisc_pkt_len to ~1 GiB (e.g. size_log=15, data=[32768]).
- Add a class with a tiny quantum of 1 and send one small packet; the
deficit loop spins billions of times under the qdisc lock and trips
the softlockup detector (panic with kernel.softlockup_panic=1).
- Reachable as root or from an unprivileged user in a fresh user+net
namespace (unshare -Urn) with namespace-local CAP_NET_ADMIN.