numa: Fix off-by-one error at MAX_CPUMASK_BITS check

Fix the CPU index check to ensure we don't go beyond the size of the
node_cpu bitmap.

CPU index is always less than MAX_CPUMASK_BITS, as documented at
sysemu.h:

> The following shall be true for all CPUs:
>   cpu->cpu_index < max_cpus <= MAX_CPUMASK_BITS

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>