svcrdma: Clean up backchannel send header encoding

Replace C structure-based XDR decoding with pointer arithmetic.
Pointer arithmetic is considered more portable.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/xprtrdma/svc_rdma_backchannel.c

@@ -200,19 +200,20 @@ rpcrdma_bc_send_request(struct svcxprt_rdma *rdma, struct rpc_rqst *rqst)
 {
 	struct rpc_xprt *xprt = rqst->rq_xprt;
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
-	struct rpcrdma_msg *headerp = (struct rpcrdma_msg *)rqst->rq_buffer;
+	__be32 *p;
 	int rc;
 
 	/* Space in the send buffer for an RPC/RDMA header is reserved
 	 * via xprt->tsh_size.
 	 */
-	headerp->rm_xid = rqst->rq_xid;
-	headerp->rm_vers = rpcrdma_version;
-	headerp->rm_credit = cpu_to_be32(r_xprt->rx_buf.rb_bc_max_requests);
-	headerp->rm_type = rdma_msg;
-	headerp->rm_body.rm_chunks[0] = xdr_zero;
-	headerp->rm_body.rm_chunks[1] = xdr_zero;
-	headerp->rm_body.rm_chunks[2] = xdr_zero;
+	p = rqst->rq_buffer;
+	*p++ = rqst->rq_xid;
+	*p++ = rpcrdma_version;
+	*p++ = cpu_to_be32(r_xprt->rx_buf.rb_bc_max_requests);
+	*p++ = rdma_msg;
+	*p++ = xdr_zero;
+	*p++ = xdr_zero;
+	*p   = xdr_zero;
 
 #ifdef SVCRDMA_BACKCHANNEL_DEBUG
 	pr_info("%s: %*ph\n", __func__, 64, rqst->rq_buffer);