cpusets: randomize node rotor used in cpuset_mem_spread_node()

From: Michal Hocko
Date: Wed Apr 13 2011 - 10:35:47 EST

I have revisited and fixed the following patch. I am keeping Jack's and
Lee's s-o-b and all others are CCed because I am not exactly sure who
worked on the patch originally.
Can we consider it for re-inclusion?