cdrom: use kmalloced buffers instead of buffers on stack

If cdrom commands are issued to a scsi drive in most cases the buffer will be
filled via dma.  This leads to bad stack corruption on non coherent platforms,
because the buffers are neither cache line aligned nor is the size a multiple
of the cache line size.  Using kmalloced buffers avoids this.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>