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/multipath.c b/drivers/md/multipath.c
index d2d3fd5..94c21c1 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -504,7 +504,7 @@
 	}
 
 	{
-		mddev->thread = md_register_thread(multipathd, mddev, "%s_multipath");
+		mddev->thread = md_register_thread(multipathd, mddev, NULL);
 		if (!mddev->thread) {
 			printk(KERN_ERR "multipath: couldn't allocate thread"
 				" for %s\n", mdname(mddev));