nfs: images -- Reserve entries for future use

These entries will be used for NFS support and
we already test partial support in vz7 version
of criu. So to make images compatible lets reserve
some of them in advance.

CC: Pavel Emelyanov <xemul@virtuozzo.com>
CC: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

images/mnt.proto

@@ -21,6 +21,11 @@ enum fstype {
 	OVERLAYFS		= 17;
 	// AUTOFS		= 18; RESERVED in criu-dev branch
 	TRACEFS			= 19;
+
+	/* These three are reserved for NFS support */
+	// RPC_PIPEFS		= 20;
+	// NFS			= 21;
+	// NFS4			= 22;
 };
 
 message mnt_entry {