md: don't delay reboot by 1 second if no MD devices exist

The md_notify_reboot() method includes a call to mdelay(1000),
to deal with "exotic SCSI devices" which are too volatile on
reboot. The delay is unconditional. Even if the machine does
not have any block devices, let alone MD devices, the kernel
shutdown sequence is slowed down.

1 second does not matter much with physical hardware, but with
certain virtualization use cases any wasted time in the bootup
& shutdown sequence counts for alot.

* drivers/md/md.c: md_notify_reboot() - only impose a delay if
  there was at least one MD device to be stopped during reboot
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -8075,12 +8075,13 @@ static int md_notify_reboot(struct notifier_block *this,
 {
 	struct list_head *tmp;
 	mddev_t *mddev;
+	int need_delay = 0;
 
 	if ((code == SYS_DOWN) || (code == SYS_HALT) || (code == SYS_POWER_OFF)) {
 
 		printk(KERN_INFO "md: stopping all md devices.\n");
 
-		for_each_mddev(mddev, tmp)
+		for_each_mddev(mddev, tmp) {
 			if (mddev_trylock(mddev)) {
 				/* Force a switch to readonly even array
 				 * appears to still be in use.  Hence
@@ -8089,13 +8090,16 @@ static int md_notify_reboot(struct notifier_block *this,
 				md_set_readonly(mddev, 100);
 				mddev_unlock(mddev);
 			}
+			need_delay = 1;
+		}
 		/*
 		 * certain more exotic SCSI devices are known to be
 		 * volatile wrt too early system reboots. While the
 		 * right place to handle this issue is the given
 		 * driver, we do want to have a safe RAID driver ...
 		 */
-		mdelay(1000*1);
+		if (need_delay)
+			mdelay(1000*1);
 	}
 	return NOTIFY_DONE;
 }