selftests: net: declare section names for bpf_offload

selftests: net: declare section names for bpf_offload

Non-ancient ip (iproute2-5.15.0, libbpf 0.7.0) refuses to load
the sample with maps because we don't generate BTF:

   libbpf: BTF is required, but is missing or corrupted.
   ERROR: opening BPF object file failed

Enable BTF by adding -g to clang flags. With that done
neither of the programs load:

  libbpf: prog 'func': error relocating .BTF.ext function info: -22
  libbpf: prog 'func': failed to relocate calls: -22
  libbpf: failed to load object 'ksft-net-drv/net/sample_ret0.bpf.o'

Andrii explains that this is because we don't specify
section names for the code. Add the section names, too.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240409031549.3531084-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>