io_uring: use rcu_dereference in io_close

Accessing the file table needs a rcu_dereference_protected().
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220518084005.3255380-7-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5372,7 +5372,8 @@ static int io_close(struct io_kiocb *req, unsigned int issue_flags)
 		spin_unlock(&files->file_lock);
 		goto err;
 	}
-	file = fdt->fd[close->fd];
+	file = rcu_dereference_protected(fdt->fd[close->fd],
+			lockdep_is_held(&files->file_lock));
 	if (!file || file->f_op == &io_uring_fops) {
 		spin_unlock(&files->file_lock);
 		file = NULL;