md/raid5: ensure we create a unique name for kmem_cache when mddev has no gendisk

We will shortly allow md devices with no gendisk (they are attached to
a dm-target instead).  That will cause mdname() to return 'mdX'.
There is one place where mdname really needs to be unique: when
creating the name for a slab cache.
So in that case, if there is no gendisk, you the address of the mddev
formatted in HEX to provide a unique name.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.h

@@ -388,7 +388,7 @@ struct raid5_private_data {
 	 * two caches.
 	 */
 	int			active_name;
-	char			cache_name[2][20];
+	char			cache_name[2][32];
 	struct kmem_cache		*slab_cache; /* for allocating stripes */
 
 	int			seq_flush, seq_write;