bcache: fix possible memory leak in bch_cached_dev_run()

memory malloced in bch_cached_dev_run() and should be freed before
leaving from the error handling cases, otherwise it will cause
memory leak.

Fixes: 0b13efec ("bcache: add return value check to bch_cached_dev_run()")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>