[IA64] Recent SPARSEMEM and DISCONTIG changes break some builds

My only objection to pfn_to_kaddr, which was introduced for HotPlug memory,
is that all arches have an identical implementation. I haven't had a chance
to pursue why yet.  There is probably some arch issue I'm unaware of.

Signed-off-by: Bob Picco <bob.picco@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>