iomap: only set iocb->private for polled bio

iocb->private is only used for polled IO, where the completer will
find the bio to poll through that field.

Assign it when we're submitting a polled bio, and get rid of the
dio->poll_bio indirection.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>