Previously, if a DirStream.Next() call would return error, we would
return the buffered data as well as the errno code in the same
request.

This would confuse the kernel, who interprets this as an invalid
response, returning syscall.EIO to the caller.

To fix this, also buffer the errno as overflow entry in the fileEntry
struct. When we encounter an error, return thus far read entries with
success. The following read contains just the error.

For testing this properly, loopbackDirStream (which reads directory
contents) must also buffer error returns.

Change-Id: Ideb8ea6f540de9189a0f366efca0db2205e5dec3