in the async socket handling routines in the cifs backend don't assume

in the async socket handling routines in the cifs backend don't assume
that conn->ntvfs_private is set correctly, instead use our copy of the
private pointer in the fde callback. This allows the cifs backend to
be used on conjunction with a NTVFS filter module.
(This used to be commit 8047b806e78815706bca81a77d04d5874ffd7d6e)