isci: unify request data structures

Make scic_sds_request a proper member of isci_request.  Also let's us
get rid of the dma pool object size tracking since we now know that all
requests are sizeof(isci_request).  While cleaning up the construct
routine incidentally replaced SCI_FIELD_OFFSET with offsetof.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/scsi/isci/host.c b/drivers/scsi/isci/host.c
index 4d0ee7b..271a7e1 100644
--- a/drivers/scsi/isci/host.c
+++ b/drivers/scsi/isci/host.c
@@ -428,14 +428,8 @@
 	if (err)
 		return err;
 
-	/*
-	 * keep the pool alloc size around, will use it for a bounds checking
-	 * when trying to convert virtual addresses to physical addresses
-	 */
-	isci_host->dma_pool_alloc_size = sizeof(struct isci_request) +
-					 sizeof(struct scic_sds_request);
 	isci_host->dma_pool = dmam_pool_create(DRV_NAME, &isci_host->pdev->dev,
-					       isci_host->dma_pool_alloc_size,
+					       sizeof(struct isci_request),
 					       SLAB_HWCACHE_ALIGN, 0);
 
 	if (!isci_host->dma_pool)