md: Improve name of threads created by md_register_thread

The management thread for raid4,5,6 arrays are all called
mdX_raid5, independent of the actual raid level, which is wrong and
can be confusion.

So change md_register_thread to use the name from the personality
unless no alternate name (like 'resync' or 'reshape') is given.

This is simpler and more correct.

Cc: Jinzc <zhenchengjin@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index ff7ed33..7efdeb4 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -2047,7 +2047,7 @@
 	conf->last_used = j;
 
 
-	mddev->thread = md_register_thread(raid1d, mddev, "%s_raid1");
+	mddev->thread = md_register_thread(raid1d, mddev, NULL);
 	if (!mddev->thread) {
 		printk(KERN_ERR
 		       "raid1: couldn't allocate thread for %s\n",