blk-cgroup: cgroup_rstat_updated() shouldn't be called on cgroup1

Currently, cgroup rstat is supported only on cgroup2 hierarchy and
rstat functions shouldn't be called on cgroup1 cgroups.  While
converting blk-cgroup core statistics to rstat, f7331648
("blk-cgroup: reimplement basic IO stats using cgroup rstat")
accidentally ended up calling cgroup_rstat_updated() on cgroup1
cgroups causing crashes.

Longer term, we probably should add cgroup1 support to rstat but for
now let's mask the call directly.

Fixes: f7331648 ("blk-cgroup: reimplement basic IO stats using cgroup rstat")
Tested-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>