fuse: don't dereference req->args on finished request

Move the check for async request after check for the request being already
finished and done with.

Reported-by: syzbot+ae0bb7aae3de6b4594e2@syzkaller.appspotmail.com
Fixes: d4993774 ("fuse: stop copying args to fuse_req")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dev.c

@@ -276,10 +276,12 @@ static void flush_bg_queue(struct fuse_conn *fc)
 void fuse_request_end(struct fuse_conn *fc, struct fuse_req *req)
 {
 	struct fuse_iqueue *fiq = &fc->iq;
-	bool async = req->args->end;
+	bool async;
 
 	if (test_and_set_bit(FR_FINISHED, &req->flags))
 		goto put_request;
+
+	async = req->args->end;
 	/*
 	 * test_and_set_bit() implies smp_mb() between bit
 	 * changing and below intr_entry check. Pairs with