pNFS/flexfiles: Fix up the ff_layout_write_pagelist failure path

If the attempt to write through pNFS fails, we need to use the same
failure semantics as for the read path: If the FF_FLAGS_NO_IO_THRU_MDS
flag is set or we have sufficient valid DSes, then we must retry through
pNFS

Fixes: d67ae825 ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>