mount: Fix cleanup_mnt_ns() when opts.root is set

In commit

    c1404f66
    mount: restore cwd after creating a roots yard (v2)

were introduced changes to the function cleanup_mnt_ns() that handle
the case when opts.root is set.

However, after commit

    2e8970be
    mount: create a mount point for the root mount namespace in the roots yard

the cleanup_mnt_ns() function always results in error:

    mnt: Can't remove the directory ...: No such file or directory

when restore fails and opts.root has been set.

Resolves #467
Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>

criu/mount.c

@@ -3263,13 +3263,10 @@ int depopulate_roots_yard(int mntns_fd, bool only_ghosts)
 
 void cleanup_mnt_ns(void)
 {
-	char path[PATH_MAX], *root = opts.root ? : "/";
-
 	if (mnt_roots == NULL)
 		return;
 
-	snprintf(path, sizeof(path), "%s/%s", root, mnt_roots);
-	if (rmdir(path))
+	if (rmdir(mnt_roots))
 		pr_perror("Can't remove the directory %s", mnt_roots);
 }