io_uring: check that we have a file table when allocating update slots

If IORING_FILE_INDEX_ALLOC is set asking for an allocated slot, the
helper doesn't check if we actually have a file table or not. The non
alloc path does do that correctly, and returns -ENXIO if we haven't set
one up.

Do the same for the allocated path, avoiding a NULL pointer dereference
when trying to find a free bit.

Fixes: a7c41b4 ("io_uring: let IORING_OP_FILES_UPDATE support choosing fixed file slots")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7973,6 +7973,9 @@ static int io_files_update_with_index_alloc(struct io_kiocb *req,
 	struct file *file;
 	int ret, fd;
 
+	if (!req->ctx->file_data)
+		return -ENXIO;
+
 	for (done = 0; done < req->rsrc_update.nr_args; done++) {
 		if (copy_from_user(&fd, &fds[done], sizeof(fd))) {
 			ret = -EFAULT;