NFSv4.1: don't send COMMIT to ds for data sync writes

Based on consensus reached in Feb 2011 interim IETF meeting regarding
use of LAYOUTCOMMIT, it has been decided that a NFS_DATA_SYNC return
from a WRITE to data server should not initiate a COMMIT.
Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/write.c

@@ -474,7 +474,10 @@ nfs_clear_request_commit(struct nfs_page *req)
 static inline
 int nfs_write_need_commit(struct nfs_write_data *data)
 {
-	return data->verf.committed != NFS_FILE_SYNC;
+	if (data->verf.committed == NFS_DATA_SYNC)
+		return data->lseg == NULL;
+	else
+		return data->verf.committed != NFS_FILE_SYNC;
 }
 
 static inline