9P: Get rid of REQ_STATUS_FLSH

This request state is mostly useless, and properly implementing it
for RDMA would require an extra lock to be taken in handle_recv()
and in rdma_cancel() to avoid this race:

    handle_recv()           rdma_cancel()
        .                     .
        .                   if req->state == SENT
    req->state = RCVD         .
        .                           req->state = FLSH

So just get rid of it.

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