io_uring: fix NULL-mm for linked reqs

__io_queue_sqe() tries to handle all request of a link,
so it's not enough to grab mm in io_sq_thread_acquire_mm()
based just on the head.

Don't check req->needs_mm and do it always.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

fs/io_uring.c

@@ -1991,10 +1991,9 @@ static void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
 	}
 }
 
-static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,
-				   struct io_kiocb *req)
+static int __io_sq_thread_acquire_mm(struct io_ring_ctx *ctx)
 {
-	if (io_op_defs[req->opcode].needs_mm && !current->mm) {
+	if (!current->mm) {
 		if (unlikely(!mmget_not_zero(ctx->sqo_mm)))
 			return -EFAULT;
 		kthread_use_mm(ctx->sqo_mm);
@@ -2003,6 +2002,14 @@ static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,
 	return 0;
 }
 
+static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,
+				   struct io_kiocb *req)
+{
+	if (!io_op_defs[req->opcode].needs_mm)
+		return 0;
+	return __io_sq_thread_acquire_mm(ctx);
+}
+
 #ifdef CONFIG_BLOCK
 static bool io_resubmit_prep(struct io_kiocb *req, int error)
 {
@@ -2781,7 +2788,7 @@ static void io_async_buf_retry(struct callback_head *cb)
 	ctx = req->ctx;
 
 	__set_current_state(TASK_RUNNING);
-	if (!io_sq_thread_acquire_mm(ctx, req)) {
+	if (!__io_sq_thread_acquire_mm(ctx)) {
 		mutex_lock(&ctx->uring_lock);
 		__io_queue_sqe(req, NULL);
 		mutex_unlock(&ctx->uring_lock);