mount: don't create a temporary directory for pivot_root()

I found this solution in the LXC code. We can open the old root, call
pivot_root(".", "."), call fchdir to the old root and call umount(".").

Now restore will not fail, if the root is read-only.
In addition it's a bit faster.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1464,7 +1464,7 @@ static int clean_mnt_ns(struct mount_info *mntinfo_tree)
 
 static int cr_pivot_root(char *root)
 {
-	char put_root[] = "crtools-put-root.XXXXXX";
+	int old_root;
 
 	pr_info("Move the root to %s\n", root ? : ".");
 
@@ -1475,34 +1475,36 @@ static int cr_pivot_root(char *root)
 		}
 	}
 
-	if (mkdtemp(put_root) == NULL) {
-		pr_perror("Can't create a temporary directory");
+	old_root = open("/", O_DIRECTORY | O_RDONLY);
+	if (old_root < 0) {
+		pr_perror("Unable to open /");
 		return -1;
 	}
 
-	if (pivot_root(".", put_root)) {
-		pr_perror("pivot_root(., %s) failed", put_root);
-		if (rmdir(put_root))
-			pr_perror("Can't remove the directory %s", put_root);
+	if (pivot_root(".", ".")) {
+		close(old_root);
+		pr_perror("pivot_root(., .) failed");
 		return -1;
 	}
 
-	if (mount("none", put_root, "none", MS_REC|MS_PRIVATE, NULL)) {
-		pr_perror("Can't remount root with MS_PRIVATE");
+	if (fchdir(old_root)) {
+		perror("Unable to change working directory");
 		return -1;
 	}
+	close(old_root);
 
-	if (mount("none", put_root, "none", MS_REC|MS_PRIVATE, NULL)) {
+	if (mount("none", ".", "none", MS_REC|MS_PRIVATE, NULL)) {
 		pr_perror("Can't remount root with MS_PRIVATE");
 		return -1;
 	}
 
-	if (umount2(put_root, MNT_DETACH)) {
-		pr_perror("Can't umount %s", put_root);
+	if (umount2(".", MNT_DETACH)) {
+		pr_perror("Can't umount the old root");
 		return -1;
 	}
-	if (rmdir(put_root)) {
-		pr_perror("Can't remove the directory %s", put_root);
+
+	if (chdir("/")) {
+		perror("Unable to change working directory");
 		return -1;
 	}