Drivers: hv: vmbus: Replace cpumask_test_cpu(, cpu_online_mask) with cpu_online()

A slight improvement in readability, and this does also remove one
memory access when NR_CPUS == 1!  ;-)
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20200617164642.37393-4-parri.andrea@gmail.comReviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/vmbus_drv.c

@@ -1716,7 +1716,7 @@ static ssize_t target_cpu_store(struct vmbus_channel *channel,
 	/* No CPUs should come up or down during this. */
 	cpus_read_lock();
 
-	if (!cpumask_test_cpu(target_cpu, cpu_online_mask)) {
+	if (!cpu_online(target_cpu)) {
 		cpus_read_unlock();
 		return -EINVAL;
 	}