restorer: add forgotten CLONE_FS flag for threads clone

Threads must share fs struct. Without this path each thread has it's own fs
struct.
Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/pie/restorer.c

@@ -1286,7 +1286,7 @@ long __export_restore_task(struct task_restore_args *args)
 	if (args->nr_threads > 1) {
 		struct thread_restore_args *thread_args = args->thread_args;
 		long clone_flags = CLONE_VM | CLONE_FILES | CLONE_SIGHAND	|
-				   CLONE_THREAD | CLONE_SYSVSEM;
+				   CLONE_THREAD | CLONE_SYSVSEM | CLONE_FS;
 		long last_pid_len;
 		long parent_tid;
 		int i, fd;