UPSTREAM: bpf: add BPF_PROG_TEST_RUN support for flow dissector

The input is packet data, the output is struct bpf_flow_key. This should
make it easy to test flow dissector programs without elaborate
setup.

Change-Id: I8639560bc8b1ce6b983365bb317b184b2cc3ab8a
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>