cpuset: fall back to cpuset.cpus on older kernels
cpuset: fall back to cpuset.cpus on older kernels

cpuset.effective_cpus is better because it does not include cpus which
we cannot use.  But it is not available on older kernels.  In that
case fall back to using cpuset.cpus and hoping for the best.

Closes #1929

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

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