scsi: sr: Don't use GFP_DMA

The allocated buffers are used as a command payload, for which the block
layer and/or DMA API do the proper bounce buffering if needed.

Link: https://lore.kernel.org/r/20211222090842.920724-1-hch@lst.de


Reported-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>