pnfs/NFSv4.1: Add multipath capabilities to pNFS flexfiles servers over NFSv3

This adds multipathing to pNFS over NFSv3 as described in the flexfiles
draft spec.
Ideally, we'd like to do the same for pNFS files, but the NFSv4.1 protocol
requires a call to EXCHANGE_ID in order to test that the connection can do
session trunking.
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pnfs_nfs.c

@@ -606,12 +606,22 @@ static int _nfs4_pnfs_v3_ds_connect(struct nfs_server *mds_srv,
 		dprintk("%s: DS %s: trying address %s\n",
 			__func__, ds->ds_remotestr, da->da_remotestr);
 
+		if (!IS_ERR(clp)) {
+			struct xprt_create xprt_args = {
+				.ident = XPRT_TRANSPORT_TCP,
+				.net = clp->cl_net,
+				.dstaddr = (struct sockaddr *)&da->da_addr,
+				.addrlen = da->da_addrlen,
+				.servername = clp->cl_hostname,
+			};
+			/* Add this address as an alias */
+			rpc_clnt_add_xprt(clp->cl_rpcclient, &xprt_args,
+					rpc_clnt_test_and_add_xprt, NULL);
+		} else
 			clp = get_v3_ds_connect(mds_srv->nfs_client,
 					(struct sockaddr *)&da->da_addr,
 					da->da_addrlen, IPPROTO_TCP,
 					timeo, retrans, au_flavor);
-		if (!IS_ERR(clp))
-			break;
 	}
 
 	if (IS_ERR(clp)) {