block: split scsi_request out of struct request

And require all drivers that want to support BLOCK_PC to allocate it
as the first thing of their private data.  To support this the legacy
IDE and BSG code is switched to set cmd_size on their queues to let
the block layer allocate the additional space.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/ide/sis5513.c

@@ -54,7 +54,7 @@
 #define DRV_NAME "sis5513"
 
 /* registers layout and init values are chipset family dependent */
-
+#undef ATA_16
 #define ATA_16		0x01
 #define ATA_33		0x02
 #define ATA_66		0x03