pNFS/filelayout: Remove the whole file layout requirement

Layout segments have been supported in pNFS for years, so remove the
requirement that the server always sends whole file layouts.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>