bcache: replace snprintf in show functions with sysfs_emit

coccicheck complains about the use of snprintf() in sysfs show functions.

Fix the following coccicheck warning:
drivers/md/bcache/sysfs.h:54:12-20: WARNING: use scnprintf or sprintf.

Implement sysfs_print() by sysfs_emit() and remove snprint() since no one
uses it any more.

Suggested-by: Coly Li <colyli@suse.de>
Signed-off-by: Qing Wang <wangqing@vivo.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20211029060930.119923-3-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>