blk-cgroup: blk_cgroup_bio_start() should use irq-safe operations on blkg->iostat_cpu

c3df5fb5 ("cgroup: rstat: fix A-A deadlock on 32bit around
u64_stats_sync") made u64_stats updates irq-safe to avoid A-A deadlocks.
Unfortunately, the conversion missed one in blk_cgroup_bio_start(). Fix it.

Fixes: 2d146aa3

 ("mm: memcontrol: switch to rstat")
Cc: stable@vger.kernel.org # v5.13+
Reported-by:  <syzbot+9738c8815b375ce482a1@syzkaller.appspotmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/YWi7NrQdVlxD6J9W@slm.duckdns.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>