preadv203: guarantee the subloop exit timely

We observed that timeout is still occasionally happening even with
introducing the tst_remaining_runtime. The likely reason is that
writer_thread needs more time in subloop.

Here exit it instantly when stop set to 1.

Signed-off-by: Li Wang <liwang@redhat.com>

testcases/kernel/syscalls/preadv2/preadv203.c

@@ -159,7 +159,7 @@ static void *writer_thread(void *unused)
 	while (!stop) {
 		int fd = fds[random() % FILES];
 
-		for (j = 0; j < CHUNKS; j++) {
+		for (j = 0; j < CHUNKS && !stop; j++) {
 			memset(buf, '0' + j, sizeof(buf));
 
 			off_t off = CHUNK_SZ * j;