NFS: Enable client side NFSv4.1 backchannel to use other transports

Forechannel transports get their own "bc_up" method to create an
endpoint for the backchannel service.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
[Anna Schumaker: Add forward declaration of struct net to xprt.h]
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/backchannel.c

@@ -7,6 +7,7 @@
 #include <linux/module.h>
 #include <linux/sunrpc/xprt.h>
 #include <linux/sunrpc/svc.h>
+#include <linux/sunrpc/svc_xprt.h>
 
 #include "xprt_rdma.h"
 
@@ -173,6 +174,26 @@ out_err:
 	return -ENOMEM;
 }
 
+/**
+ * xprt_rdma_bc_up - Create transport endpoint for backchannel service
+ * @serv: server endpoint
+ * @net: network namespace
+ *
+ * The "xprt" is an implied argument: it supplies the name of the
+ * backchannel transport class.
+ *
+ * Returns zero on success, negative errno on failure
+ */
+int xprt_rdma_bc_up(struct svc_serv *serv, struct net *net)
+{
+	int ret;
+
+	ret = svc_create_xprt(serv, "rdma-bc", net, PF_INET, 0, 0);
+	if (ret < 0)
+		return ret;
+	return 0;
+}
+
 /**
  * rpcrdma_bc_marshal_reply - Send backwards direction reply
  * @rqst: buffer containing RPC reply data