orangefs: free readdir buffer index before the dir_emit loop

We only need it while the service operation is actually in progress
since it is only used to co-ordinate the client-core's memory use. The
kernel allocates its own space.

Also clean up some comments which mislead the reader into thinking
the readdir buffers are shared memory.

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