[IA64] Avoid array overflow if there are too many cpus in SRAT table

acpi_numa_init() has to parse the whole SRAT table, even if the
kernel wants to limit the number of cpus it will use (because the
ones it is going to use may be described by entries at the end of
the SRAT table).  Avoid overflowing the node_cpuid array.

Reported-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>