Modify the NFSv4.1/pNFS client to ask for a maximum length of layout.

The code specified the length of a layout as INT64_MAX instead of
UINT64_MAX. This could result in getting a layout for less than the
full file for extremely large files. Although having little practical
effect, this patch corrects this in the code.
Detected during recent testing of the pNFS server.

MFC after: 2 weeks