bpf: Reject redirect helpers without a bpf_net_context

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

bpf: Reject redirect helpers without a bpf_net_context

The bpf_redirect*() helpers and skb_do_redirect() obtain the per-task
bpf_redirect_info via bpf_net_ctx_get_ri(), which dereferences the
current->bpf_net_context unconditionally. That context is established
on the paths that run tc BPF such as sch_handle_{ingress,egress}(),
*except* for the case where {cls,act}_bpf was attached to a proper
qdisc. A program running from there reaches the NULL deref in two ways:

* It calls bpf_redirect() directly, which dereferences the context at
the top of the helper:

tc qdisc add dev eth0 root handle 1: red limit 1MB min 10KB max 20KB \
avpkt 1000 burst 100 qevent early_drop block 10
tc filter add block 10 pref 1 bpf obj redirect.o

* It simply returns TC_ACT_REDIRECT without helper call: tcf_qevent_handle()
then dispatches to skb_do_redirect(), which dereferences the context

Rather than extending bpf_net_context management into the qdisc path,
make the redirect helpers refuse to operate when no context exists, and
have tcf_qevent_handle() drop a TC_ACT_REDIRECT verdict instead of
calling skb_do_redirect(). Previous behaviour was a crash, so nothing
regresses by not supporting it.