bpf: Fix check_return_code to only allow [0,1] in trace_iter progs

As per 15d83c4 ("bpf: Allow loading of a bpf_iter program") we only
allow a range of [0,1] for return codes. Therefore BPF_TRACE_ITER relies
on the default tnum_range(0, 1) which is set in range var. On recent merge
of net into net-next commit e92888c ("bpf: Enforce returning 0 for
fentry/fexit progs") got pulled in and caused a merge conflict with the
changes from 15d83c4. The resolution had a snall hiccup in that it
removed the [0,1] range restriction again so that BPF_TRACE_ITER would
have no enforcement. Fix it by adding it back.

Fixes: da07f52 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -7120,10 +7120,11 @@ static int check_return_code(struct bpf_verifier_env *env)
 		case BPF_TRACE_FEXIT:
 			range = tnum_const(0);
 			break;
-		case BPF_TRACE_ITER:
 		case BPF_TRACE_RAW_TP:
 		case BPF_MODIFY_RETURN:
 			return 0;
+		case BPF_TRACE_ITER:
+			break;
 		default:
 			return -ENOTSUPP;
 		}