Tests various percpu operations.

Enable with CONFIG_PERCPU_TEST=m.
Signed-off-by: NGreg Thelen <gthelen@google.com>
Acked-by: NTejun Heo <tj@kernel.org>
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>