fixed generic page.h for non-zero PAGE_OFFSET

asm-generic/page.h had several problems when used with a non-zero PAGE_OFFSET.
This patch adds a default ARCH_PFN_OFFSET and fixes the __va, __pa, and
pfn_valid macros to work with non-zero PAGE_OFFSETs.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/page.h

@@ -71,10 +71,14 @@ extern unsigned long memory_end;
 #define PAGE_OFFSET		(0)
 #endif
 
+#ifndef ARCH_PFN_OFFSET
+#define ARCH_PFN_OFFSET		(PAGE_OFFSET >> PAGE_SHIFT)
+#endif
+
 #ifndef __ASSEMBLY__
 
-#define __va(x) ((void *)((unsigned long)(x) + PAGE_OFFSET))
-#define __pa(x) ((unsigned long) (x) - PAGE_OFFSET)
+#define __va(x) ((void *)((unsigned long) (x)))
+#define __pa(x) ((unsigned long) (x))
 
 #define virt_to_pfn(kaddr)	(__pa(kaddr) >> PAGE_SHIFT)
 #define pfn_to_virt(pfn)	__va((pfn) << PAGE_SHIFT)
@@ -86,7 +90,7 @@ extern unsigned long memory_end;
 #define page_to_phys(page)      ((dma_addr_t)page_to_pfn(page) << PAGE_SHIFT)
 #endif
 
-#define pfn_valid(pfn)		((pfn) < max_mapnr)
+#define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && ((pfn) - ARCH_PFN_OFFSET) < max_mapnr)
 
 #define	virt_addr_valid(kaddr)	(((void *)(kaddr) >= (void *)PAGE_OFFSET) && \
 				((void *)(kaddr) < (void *)memory_end))