samples: bpf: Remove bpf_debug macro in favor of bpf_printk

ibumad example was implementing the bpf_debug macro which is exactly the
same as the bpf_printk macro available in bpf_helpers.h. This change
makes use of bpf_printk instead of bpf_debug.

Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>