bpftool: use new API for attaching XDP program

Switch to new bpf_xdp_attach() API to avoid deprecation warnings.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220120061422.2710637-3-andrii@kernel.org


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