ia64: Reduce stack usage by iterating over nodemask

ia64: Reduce stack usage by iterating over nodemask

GCC complains about sn2_global_tlb_purge() because of the large stack
required by the function,

  arch/ia64/sn/kernel/sn2/sn2_smp.c: In function 'sn2_global_tlb_purge':
  arch/ia64/sn/kernel/sn2/sn2_smp.c:319:1: warning: the frame size of 2176 bytes is larger than 2048 bytes [-Wframe-larger-than=]

2048 bytes of the stack are consumed by the node ID array 'nasids[]'.
But we don't actually need to put the ID array on the stack and can
use nodemask operations.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Tony Luck <tony.luck@intel.com>