sparc64: Define PAGE_OFFSET in terms of physical address bits.

This makes clearer the implications for a given choosen
value.

Based upon patches by Bob Picco.
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Bob Picco <bob.picco@oracle.com>

arch/sparc/include/asm/page_64.h

@@ -117,7 +117,9 @@ typedef pte_t *pgtable_t;
 /* We used to stick this into a hard-coded global register (%g4)
  * but that does not make sense anymore.
  */
-#define PAGE_OFFSET		_AC(0xFFFFF80000000000,UL)
+#define MAX_SUPPORTED_PA_BITS	43
+#define PAGE_OFFSET_BY_BITS(X)	(-(_AC(1,UL) << (X)))
+#define PAGE_OFFSET		PAGE_OFFSET_BY_BITS(MAX_SUPPORTED_PA_BITS)
 
 #ifndef __ASSEMBLY__