blkio-throttle: Fix possible multiplication overflow in iops calculations

o User can specify max iops value of 32bit (UINT_MAX), through cgroup
  interface. If a user has specified say 4294967294 (UNIT_MAX  - 2), then
  on 32bit platform, following multiplication can overflow.

  io_allowed = (tg->iops[rw] * jiffy_elapsed_rnd)

o Explicitly cast the multiplication to 64bit and then perform division and
  then check whether result is still great then UNINT_MAX.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-throttle.c

@@ -430,6 +430,7 @@ static bool tg_with_in_iops_limit(struct throtl_data *td, struct throtl_grp *tg,
 	bool rw = bio_data_dir(bio);
 	unsigned int io_allowed;
 	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
+	u64 tmp;
 
 	jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
 
@@ -439,7 +440,20 @@ static bool tg_with_in_iops_limit(struct throtl_data *td, struct throtl_grp *tg,
 
 	jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);
 
-	io_allowed = (tg->iops[rw] * jiffy_elapsed_rnd) / HZ;
+	/*
+	 * jiffy_elapsed_rnd should not be a big value as minimum iops can be
+	 * 1 then at max jiffy elapsed should be equivalent of 1 second as we
+	 * will allow dispatch after 1 second and after that slice should
+	 * have been trimmed.
+	 */
+
+	tmp = (u64)tg->iops[rw] * jiffy_elapsed_rnd;
+	do_div(tmp, HZ);
+
+	if (tmp > UINT_MAX)
+		io_allowed = UINT_MAX;
+	else
+		io_allowed = tmp;
 
 	if (tg->io_disp[rw] + 1 <= io_allowed) {
 		if (wait)