locktorture: Check the correct variable for allocation failure

There is a typo so this checks the wrong variable.  "chains" plural vs
"chain" singular.  We already know that "chains" is non-zero.

Fixes: 7f993623

 ("locktorture: Add call_rcu_chains module parameter")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>