bcache: replace printk() by pr_*() routines

There are still many places in bcache use printk to display kernel
message, which are suggested to be preplaced by pr_*() routines like
pr_err(), pr_info(), or pr_notice().

This patch replaces all printk() with a proper pr_*() routine for
bcache code.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>