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:
0b8917813569, b9ad010c9da9, 0142886679b3, a15d5f33f4a5, a9a0f3d93bbd,
0dd0b96bf370, 86ce62d71fb4, 7bfbe6d7027a, 985e428f4a2b, 49ae7e1d3ee1,
375bf436a766, ce2bce2e574f, e53bd8565015, 09b0b247587b

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