blkcg: avoid free blkcg_root when failed to alloc blkcg policy

this patch fix two errors, firstly avoid kfree blk_root, secondly not
free(blkcg) ,if blkcg alloc fail(blkcg == NULL), just unlock that mutex;

Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>