block: cleanup hd_struct freeing

Move hd_ref_init out of line as there it isn't anywhere near a fast path,
and rename the rcu ref freeing callbacks to be more descriptive.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>