block: remove ->poll_fn

This was intended to support users like nvme multipath, but is just
getting in the way and adding another indirect call.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>