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>
diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index 44a81e4..dc47067 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c
@@ -1306,6 +1306,17 @@
 		xs_tcp_read_reply(xprt, desc) :
 		xs_tcp_read_callback(xprt, desc);
 }
+
+static int xs_tcp_bc_up(struct svc_serv *serv, struct net *net)
+{
+	int ret;
+
+	ret = svc_create_xprt(serv, "tcp-bc", net, PF_INET, 0,
+			      SVC_SOCK_ANONYMOUS);
+	if (ret < 0)
+		return ret;
+	return 0;
+}
 #else
 static inline int _xs_tcp_read_data(struct rpc_xprt *xprt,
 					struct xdr_skb_reader *desc)
@@ -2582,6 +2593,7 @@
 	.inject_disconnect	= xs_inject_disconnect,
 #ifdef CONFIG_SUNRPC_BACKCHANNEL
 	.bc_setup		= xprt_setup_bc,
+	.bc_up			= xs_tcp_bc_up,
 	.bc_free_rqst		= xprt_free_bc_rqst,
 	.bc_destroy		= xprt_destroy_bc,
 #endif