NFS: Make "port=" mount option optional for RDMA mounts

Having to specify "proto=rdma,port=20049" is cumbersome.

RFC 8267 Section 6.3 requires NFSv4 clients to use "the alternative
well-known port number", which is 20049. Make the use of the well-
known port number automatic, just as it is for NFS/TCP and port
2049.

For NFSv2/3, Section 4.2 allows clients to simply choose 20049 as
the default or use rpcbind. I don't know of an NFS/RDMA server
implementation that registers it's NFS/RDMA service with rpcbind,
so automatically choosing 20049 seems like the better choice. The
other widely-deployed NFS/RDMA client, Solaris, also uses 20049
as the default port.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/super.c

@@ -2168,6 +2168,9 @@ static int nfs_validate_text_mount_data(void *options,
 
 	if (args->version == 4) {
 #if IS_ENABLED(CONFIG_NFS_V4)
+		if (args->nfs_server.protocol == XPRT_TRANSPORT_RDMA)
+			port = NFS_RDMA_PORT;
+		else
 			port = NFS_PORT;
 		max_namelen = NFS4_MAXNAMLEN;
 		max_pathlen = NFS4_MAXPATHLEN;
@@ -2178,8 +2181,11 @@ static int nfs_validate_text_mount_data(void *options,
 #else
 		goto out_v4_not_compiled;
 #endif /* CONFIG_NFS_V4 */
-	} else
+	} else {
 		nfs_set_mount_transport_protocol(args);
+		if (args->nfs_server.protocol == XPRT_TRANSPORT_RDMA)
+			port = NFS_RDMA_PORT;
+	}
 
 	nfs_set_port(sap, &args->nfs_server.port, port);