[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>

include/asm-ia64/page.h

@@ -120,6 +120,7 @@ extern unsigned long max_low_pfn;
 
 #define page_to_phys(page)	(page_to_pfn(page) << PAGE_SHIFT)
 #define virt_to_page(kaddr)	pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
+#define pfn_to_kaddr(pfn)	__va((pfn) << PAGE_SHIFT)
 
 typedef union ia64_va {
 	struct {