syscall: fix skip condition in skipUnprivilegedUserClone

This is a follow up CL of CL 180877:
It will skip test create user namespaces under 3 conditions:

1. sysctl file is missing
2. file reads nothing
3. user don't have permission to create namespaces

Change-Id: I25f00a6b67213bf98d654972388637789978e1fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/180937
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/syscall/exec_linux_test.go

@@ -46,7 +46,7 @@ func skipUnprivilegedUserClone(t *testing.T) {
 	// Skip the test if the sysctl that prevents unprivileged user
 	// from creating user namespaces is enabled.
 	data, errRead := ioutil.ReadFile("/proc/sys/kernel/unprivileged_userns_clone")
-	if errRead != nil || len(data) < 1 && data[0] == '0' {
+	if errRead != nil || len(data) < 1 || data[0] == '0' {
 		t.Skip("kernel prohibits user namespace in unprivileged process")
 	}
 }