Btrfs: Rev the disk format for the new back reference format

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -39,7 +39,7 @@ extern struct kmem_cache *btrfs_bit_radix_cachep;
 extern struct kmem_cache *btrfs_path_cachep;
 struct btrfs_ordered_sum;
 
-#define BTRFS_MAGIC "_B9RfS_M"
+#define BTRFS_MAGIC "_BBRfS_M"
 
 #define BTRFS_ACL_NOT_CACHED    ((void *)-1)