rcutorture: Specify CONFIG_CPUMASK_OFFSTACK=y for TREE07

This commit specifies offstack cpumasks in TREE07 in order to catch
references to unallocated cpumask_var_t variables.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

tools/testing/selftests/rcutorture/configs/rcu/TREE07

 CONFIG_SMP=y
 CONFIG_NR_CPUS=16
+CONFIG_CPUMASK_OFFSTACK=y
 CONFIG_PREEMPT_NONE=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n