pNFS/filelayout: Specify the layout segment range in LAYOUTGET

Move from only requesting full file layout segments to requesting layout
segments that match our I/O size. This means the server is still free to
return a full file layout if it wants, but partial layouts will no
longer cause an error.

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