io_u: Using initialized local variable

ddir was feed with io_u->ddir at init time but didn't got used before
reassign it to the same value or another one.

That was making this init useless.

This patch simplify this portion of code by using the initialized
variable.

io_u.c

@@ -426,12 +426,10 @@ static unsigned int __get_next_buflen(struct thread_data *td, struct io_u *io_u,
 	unsigned int minbs, maxbs;
 	unsigned long r, rand_max;
 
-	assert(ddir_rw(io_u->ddir));
+	assert(ddir_rw(ddir));
 
 	if (td->o.bs_is_seq_rand)
 		ddir = is_random ? DDIR_WRITE: DDIR_READ;
-	else
-		ddir = io_u->ddir;
 
 	minbs = td->o.min_bs[ddir];
 	maxbs = td->o.max_bs[ddir];