9pnet: allow making incomplete read requests

A user doesn't necessarily want to wait for all the requested data to
be available, since the waiting time for each request is unbounded.

The new method permits sending one read request at a time and getting
the response ASAP, allowing to use 9pnet with synthetic file systems
representing arbitrary data streams.

Link: http://lkml.kernel.org/r/20200205204053.12751-1-l29ah@cock.li


Signed-off-by: Sergey Alirzaev <l29ah@cock.li>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>