bcache: add comments for closure_fn to be called in closure_queue()

Add code comments to explain which call back function might be called
for the closure_queue(). This is an effort to make code to be more
understandable for readers.

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>