md: don't clear MD_CHANGE_CLEAN in md_update_sb() for external arrays

If this bit is cleared in md_update_sb() the kernel will allow writes to the
array if userspace triggers md_allow_write(), e.g. through stripe_cache_size,
when mdmon is not active.  When mdmon is active the array transitions to
active-idle bypassing write-pending, setting up a race for mdmon to set the
array clean before a write arrives.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -2167,9 +2167,10 @@ static void md_update_sb(mddev_t * mddev, int force_change)
 				rdev->recovery_offset = mddev->curr_resync_completed;
 
 	}	
-	if (mddev->external || !mddev->persistent) {
+	if (!mddev->persistent) {
+		if (!mddev->external)
+			clear_bit(MD_CHANGE_CLEAN, &mddev->flags);
 		clear_bit(MD_CHANGE_DEVS, &mddev->flags);
-		clear_bit(MD_CHANGE_CLEAN, &mddev->flags);
 		wake_up(&mddev->sb_wait);
 		return;
 	}