SUNRPC: Add a transport-specific private field in rpc_rqst

Currently there's a hidden and indirect mechanism for finding the
rpcrdma_req that goes with an rpc_rqst. It depends on getting from
the rq_buffer pointer in struct rpc_rqst to the struct
rpcrdma_regbuf that controls that buffer, and then to the struct
rpcrdma_req it goes with.

This was done back in the day to avoid the need to add a per-rqst
pointer or to alter the buf_free API when support for RPC-over-RDMA
was introduced.

I'm about to change the way regbuf's work to support larger inline
thresholds. Now is a good time to replace this indirect mechanism
with something that is more straightforward. I guess this should be
considered a clean up.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/backchannel.c

@@ -55,11 +55,9 @@ static int rpcrdma_bc_setup_rqst(struct rpcrdma_xprt *r_xprt,
 	rb = rpcrdma_alloc_regbuf(ia, size, GFP_KERNEL);
 	if (IS_ERR(rb))
 		goto out_fail;
-	rb->rg_owner = req;
 	req->rl_sendbuf = rb;
-	/* so that rpcr_to_rdmar works when receiving a request */
-	rqst->rq_buffer = (void *)req->rl_sendbuf->rg_base;
-	xdr_buf_init(&rqst->rq_snd_buf, rqst->rq_buffer, size);
+	xdr_buf_init(&rqst->rq_snd_buf, rb->rg_base, size);
+	rpcrdma_set_xprtdata(rqst, req);
 	return 0;
 
 out_fail: