bpf: add a get_helper_proto() utility function

Extract the part of check_helper_call() as a utility function allowing
to query 'struct bpf_func_proto' for a specific helper function id.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240722233844.1406874-2-eddyz87@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>