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>

drivers/scsi/isci/host.h

@@ -82,7 +82,6 @@ struct isci_host {
 	struct list_head timers;
 	void *core_ctrl_memory;
 	struct dma_pool *dma_pool;
-	unsigned int dma_pool_alloc_size;
 	struct isci_phy phys[SCI_MAX_PHYS];
 	struct isci_port ports[SCI_MAX_PORTS + 1]; /* includes dummy port */
 	struct sas_ha_struct sas_ha;