gdrom: missed conversion from req->buffer

The friendly Intel kbuild test robot reported:

drivers/cdrom/gdrom.c: In function 'gdrom_readdisk_dma':
drivers/cdrom/gdrom.c:605:3: error: 'struct request' has no member named 'buffer'

Convert that from req->buffer to bio_data(rq->bio). Apparently
my grep missed this one, and I don't build for Sega Dreamcast
enough.

Signed-off-by: Jens Axboe <axboe@fb.com>