xprtrdma: Simplify rpcrdma_convert_kvec() and frwr_map()

Clean up.

Remove a conditional branch from the SGL set-up loop in frwr_map():
Instead of using either sg_set_page() or sg_set_buf(), initialize
the mr_page field properly when rpcrdma_convert_kvec() converts the
kvec to an SGL entry. frwr_map() can then invoke sg_set_page()
unconditionally.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/frwr_ops.c

@@ -306,14 +306,10 @@ struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
 	if (nsegs > ep->re_max_fr_depth)
 		nsegs = ep->re_max_fr_depth;
 	for (i = 0; i < nsegs;) {
-		if (seg->mr_page)
-			sg_set_page(&mr->mr_sg[i],
-				    seg->mr_page,
-				    seg->mr_len,
-				    offset_in_page(seg->mr_offset));
-		else
-			sg_set_buf(&mr->mr_sg[i], seg->mr_offset,
-				   seg->mr_len);
+		sg_set_page(&mr->mr_sg[i],
+			    seg->mr_page,
+			    seg->mr_len,
+			    offset_in_page(seg->mr_offset));
 
 		++seg;
 		++i;

net/sunrpc/xprtrdma/rpc_rdma.c

@@ -213,7 +213,7 @@ static struct rpcrdma_mr_seg *
 rpcrdma_convert_kvec(struct kvec *vec, struct rpcrdma_mr_seg *seg,
 		     unsigned int *n)
 {
-	seg->mr_page = NULL;
+	seg->mr_page = virt_to_page(vec->iov_base);
 	seg->mr_offset = vec->iov_base;
 	seg->mr_len = vec->iov_len;
 	++seg;

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -283,10 +283,11 @@ enum {
 				  RPCRDMA_MAX_IOV_SEGS,
 };
 
-struct rpcrdma_mr_seg {		/* chunk descriptors */
-	u32		mr_len;		/* length of chunk or segment */
-	struct page	*mr_page;	/* owning page, if any */
-	char		*mr_offset;	/* kva if no page, else offset */
+/* Arguments for DMA mapping and registration */
+struct rpcrdma_mr_seg {
+	u32		mr_len;		/* length of segment */
+	struct page	*mr_page;	/* underlying struct page */
+	char		*mr_offset;	/* IN: page offset, OUT: iova */
 };
 
 /* The Send SGE array is provisioned to send a maximum size