[OpenAFS] Cache file status after Network error

[Harald Barth]

> A write error might be caused by a failure in the communication
> path between the client and the file server.  Such a failure should not
> result in the data being discarded from the cache.  

So we just told the user (by returning error on the close) that the
file is bad, but subsequent reads should proceed as everything went
fine? If we can guarantee to obain consistency at a later point, the
close should not have returned failiure in the first place. You can't
have it both ways. Until someone implements disconnected AFS and fixes
the writeback later, I suggest to throw away the data which never will
make it to the server.

Harald.