selftests/bpf: use thoff instead of nhoff in BPF flow dissector

We are returning thoff from the flow dissector, not the nhoff. Pass
thoff along with nhoff to the bpf program (initially thoff == nhoff)
and expect flow dissector amend/return thoff, not nhoff.

This avoids confusion, when by the time bpf flow dissector exits,
nhoff == thoff, which doesn't make much sense.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/core/flow_dissector.c

@@ -783,6 +783,7 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 		/* Pass parameters to the BPF program */
 		cb->qdisc_cb.flow_keys = &flow_keys;
 		flow_keys.nhoff = nhoff;
+		flow_keys.thoff = nhoff;
 
 		bpf_compute_data_pointers((struct sk_buff *)skb);
 		result = BPF_PROG_RUN(attached, skb);