block: simplify struct io_context refcounting

Don't hold a reference to ->refcount for each active reference, but
just one for all active references.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20211209063131.18537-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>