pNFS/flexfiles: Specify the layout segment range in LAYOUTGET

Move from requesting only 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, but we will no longer
error out if it does not.

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