NFS: Introduce generic mount client API

For NFSv2 and v3 mounts, the first step is to contact the server's MOUNTD
and request the file handle for the root of the mounted share.  Add a
function to the NFS client that handles this operation.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -991,6 +991,63 @@ static int nfs_parse_mount_options(char *raw,
 	return 0;
 }
 
+/*
+ * Use the remote server's MOUNT service to request the NFS file handle
+ * corresponding to the provided path.
+ */
+static int nfs_try_mount(struct nfs_parsed_mount_data *args,
+			 struct nfs_fh *root_fh)
+{
+	struct sockaddr_in sin;
+	int status;
+
+	if (args->mount_server.version == 0) {
+		if (args->flags & NFS_MOUNT_VER3)
+			args->mount_server.version = NFS_MNT3_VERSION;
+		else
+			args->mount_server.version = NFS_MNT_VERSION;
+	}
+
+	/*
+	 * Construct the mount server's address.
+	 */
+	if (args->mount_server.address.sin_addr.s_addr != INADDR_ANY)
+		sin = args->mount_server.address;
+	else
+		sin = args->nfs_server.address;
+	if (args->mount_server.port == 0) {
+		status = rpcb_getport_sync(&sin,
+					   args->mount_server.program,
+					   args->mount_server.version,
+					   args->mount_server.protocol);
+		if (status < 0)
+			goto out_err;
+		sin.sin_port = htons(status);
+	} else
+		sin.sin_port = htons(args->mount_server.port);
+
+	/*
+	 * Now ask the mount server to map our export path
+	 * to a file handle.
+	 */
+	status = nfs_mount((struct sockaddr *) &sin,
+			   sizeof(sin),
+			   args->nfs_server.hostname,
+			   args->nfs_server.export_path,
+			   args->mount_server.version,
+			   args->mount_server.protocol,
+			   root_fh);
+	if (status < 0)
+		goto out_err;
+
+	return status;
+
+out_err:
+	dfprintk(MOUNT, "NFS: unable to contact server on host "
+		 NIPQUAD_FMT "\n", NIPQUAD(sin.sin_addr.s_addr));
+	return status;
+}
+
 /*
  * Validate the NFS2/NFS3 mount data
  * - fills in the mount root filehandle