samples/bpf: adapt samples/bpf to bpf_xdp_xxx() APIs

Use new bpf_xdp_*() APIs across all XDP-related BPF samples.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220120061422.2710637-5-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>