samples/bpf: Xdp_redirect_cpu fix missing tracepoint attach

When sample xdp_redirect_cpu was converted to use libbpf, the
tracepoints used by this sample were not getting attached automatically
like with bpf_load.c. The BPF-maps was still getting loaded, thus
nobody notice that the tracepoints were not updating these maps.

This fix doesn't use the new skeleton code, as this bug was introduced
in v5.1 and stable might want to backport this. E.g. Red Hat QA uses
this sample as part of their testing.

Fixes: bbaf6029 ("samples/bpf: Convert XDP samples to libbpf usage")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157685877642.26195.2798780195186786841.stgit@firesoul