samples/bpf: xdp_redirect, correctly get dummy program id

samples/bpf: xdp_redirect, correctly get dummy program id

When we terminate xdp_redirect, it ends up with following message:
"Program on iface OUT changed, not removing"
This results in dummy prog still attached to OUT interface.
It is because signal handler checks if the programs are the same that
we had attached. But while fetching dummy_prog_id, current code uses
prog_fd instead of dummy_prog_fd. This patch passes the correct fd.

Fixes: 3b7a8ec2dec3 ("samples/bpf: Check the prog id before exiting")
Signed-off-by: Prashant Bhole <prashantbhole.linux@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>