lightnvm: move metadata mapping to lower level driver

Now that blk_rq_map_kern can map both kmem and vmem, move internal
metadata mapping down to the lower level driver.

Reviewed-by: Javier González <javier@javigon.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hans Holmberg <hans@owltronix.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>