bpf: Switch BPF_F_KPROBE_MULTI_RETURN macro to enum

bpf: Switch BPF_F_KPROBE_MULTI_RETURN macro to enum

Switching BPF_F_KPROBE_MULTI_RETURN macro to anonymous enum,
so it'd show up in vmlinux.h. There's not functional change
compared to having this as macro.

Acked-by: Yafang Shao <laoar.shao@gmail.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230809083440.3209381-2-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>