bcache: remove redundant variables i and n

Variables i and n are being assigned but are never used. They are
redundant and can be removed.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Coly Li <colyli@suse.de>
Addresses-Coverity: ("Unused value")
Signed-off-by: Jens Axboe <axboe@kernel.dk>