Do not fail if /tmp does not exist

Currently if /tmp does not exist, CRIU fails because it will not be
able to create a temporary directory there.  But when checkpointing
and restoring containers, we cannot rely on the existence of /tmp.
For such containers, we should use root (/).  The temporary directory
will be removed after CRIU is done.
Signed-off-by: Saied Kazemi <saied@google.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -774,7 +774,9 @@ int open_mount(unsigned int s_dev)
 static int open_mountpoint(struct mount_info *pm)
 {
 	int fd = -1, ns_old = -1;
-	char mnt_path[] = "/tmp/cr-tmpfs.XXXXXX";
+	char mnt_path_tmp[] = "/tmp/cr-tmpfs.XXXXXX";
+	char mnt_path_root[] = "/cr-tmpfs.XXXXXX";
+	char *mnt_path = mnt_path_tmp;
 	int cwd_fd;
 
 	/*
@@ -803,7 +805,10 @@ static int open_mountpoint(struct mount_info *pm)
 	if (switch_ns(root_item->pid.real, &mnt_ns_desc, &ns_old) < 0)
 		goto out;
 
-	if (mkdtemp(mnt_path) == NULL) {
+	mnt_path = mkdtemp(mnt_path_tmp);
+	if (mnt_path == NULL && errno == ENOENT)
+		mnt_path = mkdtemp(mnt_path_root);
+	if (mnt_path == NULL) {
 		pr_perror("Can't create a temporary directory");
 		goto out;
 	}