bpf: fix inconsistent return types of bpf_xdp_copy_buf().

Fix inconsistent return types in two implementations of bpf_xdp_copy_buf().

There are two implementations: one is an empty implementation whose return
type does not match the actual implementation.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230804005101.1534505-1-thinker.li@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>