There are two cases for cr_fdset_open

 - It might be called with already allocated
   memory so we should reuse it.

 - It might be called with NULL pointing out
   that caller expects us to allocate memory.

If an open() error happens somewhere inside cr_fdset_open
it requires two error paths

 - Just close all files opened but don't free memory
   if it was not allocated by us

 - Close all files opened *and* free memory allocated
   by us.

In any case we should close all files opened so close_cr_fdset()
helper is splitted into two parts.

Also the caller should be ready for such semantics as well and
do not re-assign pointers obtained but simply test for NULL
on results.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>