Btrfs: fix error handling in __add_reloc_root()

We dereferenced "node" in the error message after freeing it.  Also
btrfs_panic() can return so we should return an error code instead of
continuing.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

fs/btrfs/relocation.c

@@ -1239,10 +1239,11 @@ static int __must_check __add_reloc_root(struct btrfs_root *root)
 			      node->bytenr, &node->rb_node);
 	spin_unlock(&rc->reloc_root_tree.lock);
 	if (rb_node) {
-		kfree(node);
 		btrfs_panic(root->fs_info, -EEXIST, "Duplicate root found "
 			    "for start=%llu while inserting into relocation "
 			    "tree\n");
+		kfree(node);
+		return -EEXIST;
 	}
 
 	list_add_tail(&root->root_list, &rc->reloc_roots);