nfs-proxy: add support for open-less read requests

in a production we observe situations, where client
decides to perform IO through MDS (door) with
all-zero stateid. Such behavior is  allowed by
rfc5661 (#8.2.3) and should be supported
to keep clients happy.

Acked-by: Gerd Behrmann
Target: master, 2.8, 2.7
Require-book: no
Require-notes: no