SUNRPC: Add a structure to track multiple transports

In order to support multipathing/trunking we will need the ability to
track multiple transports. This patch sets up a basic structure for
doing so.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprt.c

@@ -1319,6 +1319,7 @@ static void xprt_init(struct rpc_xprt *xprt, struct net *net)
 	spin_lock_init(&xprt->bc_pa_lock);
 	INIT_LIST_HEAD(&xprt->bc_pa_list);
 #endif /* CONFIG_SUNRPC_BACKCHANNEL */
+	INIT_LIST_HEAD(&xprt->xprt_switch);
 
 	xprt->last_used = jiffies;
 	xprt->cwnd = RPC_INITCWND;