Fix double close issue in diskfile

The DiskWriter was closing the file descriptor when it finished
writing but initializing it to None.  Therefore, the DiskFile
context manager would then try to close it also.

Change-Id: I188ec814d025e28c7b89532f0502ebf1d4a20a09
Signed-off-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/6317
Reviewed-by: Peter Portante <pportant@redhat.com>
Reviewed-by: pushpesh sharma <psharma@redhat.com>
Tested-by: pushpesh sharma <psharma@redhat.com>
Signed-off-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/6487