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>