[IA64] Fix fast syscall version of getcpu()

GETCPU(2) says:
  int getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache);
  ...
  When either cpu or node is NULL nothing is written to the respective pointer.

But the fast system call path had no checks for NULL, and would
thus return -EFAULT if either (or both) of these were NULL.

Reported-by: Mike Frysinger <vapier@gentoo.org>
Tested-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>