IB/{hfi1,qib}: Cleanup open coded sge sizing

Sge sizing is done in several places using an open coded method.

This can cause maintenance issues.  The open coded method is
encapsulated in a helper routine.  The helper was introduced with
commit:

1198fcea8a ("IB/hfi1, rdmavt: Move SGE state helper routines into
rdmavt")

Update all call sites that have the open coded path with the helper
routine.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/qib/qib_ud.c

@@ -172,12 +172,8 @@ static void qib_ud_loopback(struct rvt_qp *sqp, struct rvt_swqe *swqe)
 	ssge.num_sge = swqe->wr.num_sge;
 	sge = &ssge.sge;
 	while (length) {
-		u32 len = sge->length;
+		u32 len = rvt_get_sge_length(sge, length);
 
-		if (len > length)
-			len = length;
-		if (len > sge->sge_length)
-			len = sge->sge_length;
 		rvt_copy_sge(qp, &qp->r_sge, sge->vaddr, len, true, false);
 		sge->vaddr += len;
 		sge->length -= len;