freezer, sched: Report frozen tasks as 'D' instead of 'R'

Before commit:

  f5d39b02 ("freezer,sched: Rewrite core freezer logic")

the frozen task stat was reported as 'D' in cgroup v1.

However, after rewriting the core freezer logic, the frozen task stat is
reported as 'R'. This is confusing, especially when a task with stat of
'S' is frozen.

This bug can be reproduced with these steps:

	$ cd /sys/fs/cgroup/freezer/
	$ mkdir test
	$ sleep 1000 &
	[1] 739         // task whose stat is 'S'
	$ echo 739 > test/cgroup.procs
	$ echo FROZEN > test/freezer.state
	$ ps -aux | grep 739
	root     739  0.1  0.0   8376  1812 pts/0    R    10:56   0:00 sleep 1000

As shown above, a task whose stat is 'S' was changed to 'R' when it was
frozen.

To solve this regression, simply maintain the same reported state as
before the rewrite.

[ mingo: Enhanced the changelog and comments ]

Fixes: f5d39b02 ("freezer,sched: Rewrite core freezer logic")
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Koutný <mkoutny@suse.com>
Link: https://lore.kernel.org/r/20241217004818.3200515-1-chenridong@huaweicloud.com