selftests/bpf: signedness bug in enable_all_controllers()

The "len" variable needs to be signed for the error handling to work
properly.

Fixes: 596092ef ("selftests/bpf: enable all available cgroup v2 controllers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>