NFS: Add in v4.2 callback operation

NFS v4.2 adds a CB_OFFLOAD operation used by COPY and WRITE_PLUS.  Since
neither of these operations have been implemented yet, simply return
NFS4ERR_NOTSUPP.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/callback.h b/fs/nfs/callback.h
index 41cf893..84326e9 100644
--- a/fs/nfs/callback.h
+++ b/fs/nfs/callback.h
@@ -32,6 +32,8 @@
 	OP_CB_WANTS_CANCELLED = 12,
 	OP_CB_NOTIFY_LOCK   = 13,
 	OP_CB_NOTIFY_DEVICEID = 14,
+/* Callback operations new to NFSv4.2 */
+	OP_CB_OFFLOAD = 15,
 	OP_CB_ILLEGAL = 10044,
 };