This commit adds the locking counterpart to rcutorture.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Make n_lock_torture_errors and torture_spinlock static
  as suggested by Fengguang Wu. ]
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>