Fix logspam from system-background cpuset.

system-background can't be opened by most processes, since system-bg is
a privileged cpuset. Remove the log message when it can't be
opened. We'll still log a failure to use the fd if we try to use it
after it can't be opened, which is fine.

bug 26249842

Change-Id: I99f185c51645bc81b6a41732848227c27675cdf7