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: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>