net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue

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

net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue

gred_enqueue(), bfifo_enqueue() and plug_enqueue() admit a packet when the
current backlog plus the packet length fits within the queue limit:

sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit (gred default VQ)
gred_backlog+qdisc_pkt_len(skb) <= q->limit (gred configured VQ)
sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit (bfifo)
sch->qstats.backlog + skb->len <= q->limit (plug)

sch->qstats.backlog and q->backlog are u32, and qdisc_pkt_len()/skb->len
are unsigned int, so all sums are computed in 32 bits and wrap at 2^32.
Once the true backlog exceeds 4 GiB the wrapped sum becomes small and
admission keeps succeeding, so the queue grows without bound and the kernel
can be driven to OOM.

Promote the sums to u64 so admission stops once the true backlog exceeds
the limit. The limit is u32, so the bounded queue stays below 2^32 and
the stored u32 backlog never wraps.

The bug can only be reproduced as root (albeit with ridiculous setup):
attach a gred (or bfifo/plug) qdisc with a limit near 4 GiB,
leaving the default VQ unconfigured (for gred), and drive >4 GiB of
queued traffic (e.g. via a size table / stab to inflate qdisc_pkt_len,
or sustained high-rate traffic). The u32 backlog+len sum wraps at 2^32,
admission keeps succeeding, and the queue grows unboundedly to OOM.