bcache: writeback rate clamping: make 32 bit safe

Sorry this got through to linux-block, was detected by the kbuilds test
robot.  NSEC_PER_SEC is a long constant; 2.5 * 10^9 doesn't fit in a
signed long constant.

Fixes: e41166c5c44e ("bcache: writeback rate shouldn't artifically clamp")
Reviewed-by: Coly Li <colyli@suse.de>
Signed-off-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/util.c

@@ -238,8 +238,8 @@ uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done)
 	 * don't let us sleep more than 2.5 seconds (so we can notice/respond
 	 * if the control system tells us to speed up!).
 	 */
-	if (time_before64(now + NSEC_PER_SEC * 5 / 2, d->next))
-		d->next = now + NSEC_PER_SEC * 5 / 2;
+	if (time_before64(now + NSEC_PER_SEC * 5LLU / 2LLU, d->next))
+		d->next = now + NSEC_PER_SEC * 5LLU / 2LLU;
 
 	if (time_after64(now - NSEC_PER_SEC * 2, d->next))
 		d->next = now - NSEC_PER_SEC * 2;