scsi: gdth: reuse dma coherent allocation in gdth_show_info

gdth_show_info currently allocs and frees a dma buffer four times which
isn't very efficient. Reuse a single allocation instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>