bcache: add return value check to bch_cached_dev_run()

This patch adds return value check to bch_cached_dev_run(), now if there
is error happens inside bch_cached_dev_run(), it can be catched.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>