blk-cgroup: remove blkg_lookup_check

The combinations of an error check with an ERR_PTR return and a lookup
with a NULL return leads to ugly handling of the return values in the
callers.  Just open coding the check and the lookup is much simpler.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andreas Herrmann <aherrmann@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220921180501.1539876-6-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>