tools/bpftool: Fix attaching flow dissector

tools/bpftool: Fix attaching flow dissector

My earlier patch to reject non-zero arguments to flow dissector attach
broke attaching via bpftool. Instead of 0 it uses -1 for target_fd.
Fix this by passing a zero argument when attaching the flow dissector.

Fixes: 1b514239e859 ("bpf: flow_dissector: Check value of unused flags to BPF_PROG_ATTACH")
Reported-by: Jiri Benc <[email protected]>
Signed-off-by: Lorenz Bauer <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]