bpf: Don't redirect packets with invalid pkt_len

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

bpf: Don't redirect packets with invalid pkt_len

Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any

skbs, that is, the flow->head is null.

The root cause, as the [2] says, is because that bpf_prog_test_run_skb()

run a bpf prog which redirects empty skbs.

So we should determine whether the length of the packet modified by bpf

prog or others like bpf_prog_test is valid before forwarding it directly.