pNFS: Remove unwanted optimisation of layoutget

If we knew that the file was empty, we wouldn't be asking for a layout.
Any optimisation here is already done before calling pnfs_update_layout().
As it stands, we sometimes end up doing an unnecessary inband read to
the MDS even when holding a layout.

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