iomap: add struct iomap_data

iomap: add struct iomap_data

We pass a lot of arguments to iomap_apply(), and subsequently to the
actors that it calls. In preparation for adding one more argument,
switch them to using a struct iomap_data instead. The actor gets a const
version of that, they are not supposed to change anything in it.

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