cgroup: stash cgroup_root reference into cgroup_fs_context

Note that this reference is *NOT* contributing to refcount of
cgroup_root in question and is valid only until cgroup_do_mount()
returns.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/cgroup/cgroup-v1.c

@@ -1208,6 +1208,7 @@ int cgroup1_get_tree(struct fs_context *fc)
 		if (root->flags ^ ctx->flags)
 			pr_warn("new mount options do not match the existing superblock, will be ignored\n");
 
+		ctx->root = root;
 		ret = 0;
 		goto out_unlock;
 	}
@@ -1234,7 +1235,8 @@ int cgroup1_get_tree(struct fs_context *fc)
 		goto out_unlock;
 	}
 
-	init_cgroup_root(root, ctx);
+	ctx->root = root;
+	init_cgroup_root(ctx);
 
 	ret = cgroup_setup_root(root, ctx->subsys_mask);
 	if (ret)