NFSv4.2/pnfs: Don't use READ_PLUS with pNFS yet

We have no way of tracking server READ_PLUS support in pNFS for now, so
just disable it.

Reported-by: "Mkrtchyan, Tigran" <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>