samples/bpf: use bpf_set_link_xdp_fd

Use bpf_set_link_xdp_fd instead of set_link_xdp_fd to remove some
code duplication and benefit of netlink ext ack errors message.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>