bcache: fix code comments style

This patch fixes 3 style issues warned by checkpatch.pl,
- Comment lines are not aligned
- Comments use "/*" on subsequent lines
- Comment lines use a trailing "*/"

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