s390/mm: add missing pfn_to_kaddr helper

kasan common code uses pfn_to_kaddr, which is defined by many other
architectures. Adding it as well to avoid a build error.
Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/page.h

@@ -161,6 +161,7 @@ static inline int devmem_is_allowed(unsigned long pfn)
 
 #define virt_to_pfn(kaddr)	(__pa(kaddr) >> PAGE_SHIFT)
 #define pfn_to_virt(pfn)	__va((pfn) << PAGE_SHIFT)
+#define pfn_to_kaddr(pfn)	pfn_to_virt(pfn)
 
 #define virt_to_page(kaddr)	pfn_to_page(virt_to_pfn(kaddr))
 #define page_to_virt(page)	pfn_to_virt(page_to_pfn(page))