UPSTREAM: 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.

Change-Id: I1ee808c190b5b472dd374f20c85a7846cfe2c9bb
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>