GRES topology bug in core selection logic fixed.

This problem would effect systems in which specific GRES are associated
with specific CPUs.
One possible result is the CPUs identified as usable could be inappropriate
and job would be held when trying to layout out the tasks on CPUs (all
done as part of the job allocation process).
The other problem is that if multiple GRES are linked to specific CPUs,
there was a CPU bitmap OR which should have been an AND, resulting in
some CPUs being identified as usable, but not available to all GRES.