orangefs: skip forward to the next directory entry if seek is short

If userspace seeks to a position in the stream which is not correct, it
would have returned EIO because the data in the buffer at that offset
would be incorrect.  This and the userspace daemon returning a corrupt
directory are indistinguishable.

Now if the data does not look right, skip forward to the next chunk and
try again.  The motivation is that if the directory changes, an
application may seek to a position that was valid and no longer is valid.

It is not yet possible for a directory to change.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>