bpf: Allow kfuncs return 'void *'

Recognize return of 'void *' from kfunc as returning unknown scalar.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20240307031228.42896-2-alexei.starovoitov@gmail.com


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