pNFS/flexfiles: ff_layout_pg_init_read should exit on error

If we get an error while retrieving the layout, then we should
report it rather than falling back to I/O through the MDS.

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