[SCSI] use dynamically allocated sense buffer

This removes static array sense_buffer in scsi_cmnd and uses
dynamically allocated sense_buffer (with GFP_DMA).

The reason for doing this is that some architectures need cacheline
aligned buffer for DMA:

http://lkml.org/lkml/2007/11/19/2

The problems are that scsi_eh_prep_cmnd puts scsi_cmnd::sense_buffer
to sglist and some LLDs directly DMA to scsi_cmnd::sense_buffer. It's
necessary to DMA to scsi_cmnd::sense_buffer safely. This patch solves
these issues.

__scsi_get_command allocates sense_buffer via kmem_cache_alloc and
attaches it to a scsi_cmnd so everything just work as before.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

include/scsi/scsi_cmnd.h

@@ -88,7 +88,7 @@ struct scsi_cmnd {
 				   	   working on */
 
 #define SCSI_SENSE_BUFFERSIZE 	96
-	unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE];
+	unsigned char *sense_buffer;
 				/* obtained by REQUEST SENSE when
 				 * CHECK CONDITION is received on original
 				 * command (auto-sense) */