blkcg: revert blkcg cleanups series

blkcg: revert blkcg cleanups series

This reverts a series committed earlier due to null pointer exception
bug report in [1]. It seems there are edge case interactions that I did
not consider and will need some time to understand what causes the
adverse interactions.

The original series can be found in [2] with a follow up series in [3].

[1] https://www.spinics.net/lists/cgroups/msg20719.html
[2] https://lore.kernel.org/lkml/20180911184137.35897-1-dennisszhou@gmail.com/
[3] https://lore.kernel.org/lkml/20181020185612.51587-1-dennis@kernel.org/

This reverts the following commits:
8a03eb552948, 5a82ddfd9c3f, d5d670a738bb, 6568804fbfcb, 7c211b29a858,
9ed1ba108f4d, c90f3d48a618, 80bbaa266b25, c9254afe624c, dc713a38e6ae,
09755b4ac246, ed28fdf602af, 021fca0c5dc1, 137d27f6a3ae

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>