cpumask: convert rcutorture.c

We're getting rid of cpumasks on the stack.

Simply change tmp_mask to a global, and allocate it in
rcu_torture_init().

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@freedesktop.org>