gdrom: support highmem

The gdrom driver only has a single reference to the virtual address of
the bio data, and uses that only to get the physical address.  Switch
to deriving the physical address from the page directly and thus avoid
bounce buffering highmem data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406061648.811275-1-hch@lst.de


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