samples/bpf: replace legacy map with the BTF-defined map

samples/bpf: replace legacy map with the BTF-defined map

With libbpf 1.0 release, support for legacy BPF map declaration syntax
had been dropped. If you run a program using legacy BPF in the latest
libbpf, the following error will be output.

    libbpf: map 'lwt_len_hist_map' (legacy): legacy map definitions are deprecated, use BTF-defined maps instead
    libbpf: Use of BPF_ANNOTATE_KV_PAIR is deprecated, use BTF-defined maps in .maps section instead

This commit replaces legacy map with the BTF-defined map.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Link: https://lore.kernel.org/r/20230115071613.125791-7-danieltimlee@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>