svcrdma: Fix backchannel return code

Way back when I was writing the RPC/RDMA server-side backchannel
code, I misread the TCP backchannel reply handler logic. When
svc_tcp_recvfrom() successfully receives a backchannel reply, it
does not return -EAGAIN. It sets XPT_DATA and returns zero.

Update svc_rdma_recvfrom() to return zero. Here, XPT_DATA doesn't
need to be set again: it is set whenever a new message is received,
behind a spin lock in a single threaded context.

Also, if handling the cb reply is not successful, the message is
simply dropped. There's no special message framing to deal with as
there is in the TCP case.

Now that the handle_bc_reply() return value is ignored, I've removed
the dprintk call sites in the error exit of handle_bc_reply() in
favor of trace points in other areas that already report the error
cases.

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

include/linux/sunrpc/svc_rdma.h

@@ -160,9 +160,8 @@ struct svc_rdma_send_ctxt {
 };
 
 /* svc_rdma_backchannel.c */
-extern int svc_rdma_handle_bc_reply(struct rpc_xprt *xprt,
-				    __be32 *rdma_resp,
-				    struct xdr_buf *rcvbuf);
+extern void svc_rdma_handle_bc_reply(struct svc_rqst *rqstp,
+				     struct svc_rdma_recv_ctxt *rctxt);
 
 /* svc_rdma_recvfrom.c */
 extern void svc_rdma_recv_ctxts_destroy(struct svcxprt_rdma *rdma);