blkcg: fix incorrect read/write sync/async stat accounting

While unifying how blkcg stats are collected, 77ea7338 ("blkcg:
move io_service_bytes and io_serviced stats into blkcg_gq")
incorrectly used bio->flags instead of bio->rw to tell the IO type.
This made IOs to be accounted as the wrong type.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 77ea7338 ("blkcg: move io_service_bytes and io_serviced stats into blkcg_gq")
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>