rcu: Fix synchronize_sched_expedited() type error for "s"

The type of "s" has been "long" rather than the correct "unsigned long"
for quite some time.  This commit fixes this type error.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3386,7 +3386,7 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
 void synchronize_sched_expedited(void)
 {
 	int cpu;
-	long s;
+	unsigned long s;
 	struct rcu_node *rnp;
 	struct rcu_state *rsp = &rcu_sched_state;