md/bcache: convert comma to semicolon

Replace a comma between expression statements by a semicolon.

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: Coly Li <colyli@sue.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>