9P/RDMA: Protect against duplicate replies

A well-behaved server would not send twice the reply to a request.
But if it ever happens...
This additional check prevents the kernel from leaking memory
and possibly more nasty consequences in that unlikely event.

Signed-off-by: Simon Derr <simon.derr@bull.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>