xen: fix for_each_cpu when NR_CPUS=1

xen: fix for_each_cpu when NR_CPUS=1

When running an hypervisor build with NR_CPUS=1 for_each_cpu does not
take into account whether the bit of the CPU is set or not in the
provided mask.

This means that whatever we have in the bodies of these loops is always
done once, even if the mask was empty and it should never be done. This
is clearly a bug and was in fact causing an assert to trigger in credit2
code.

Removing the special casing of NR_CPUS == 1 makes things work again.

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>