bpf: Unify and simplify btf_func_proto_check error handling

Replace 'err = x; break;' with 'return x;'.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221124002838.2700179-1-sdf@google.com