bcache: Add comments for blkdev_put() in registration code path

Add comments to explain why in register_bcache() blkdev_put() won't
be called in two location. Add comments to explain why blkdev_put()
must be called in register_cache() when cache_alloc() failed.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>