net/flow_dissector: move bpf case into __skb_flow_bpf_dissect

This way, we can reuse it for flow dissector in BPF_PROG_TEST_RUN.

No functional changes.

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