sparc64: Define WANT_PAGE_VIRTUAL

As sparse warns, without this struct page pointer subtraction is
extremely expensive, and this is a pretty common operation in
fast paths.

With this define struct page becomes 64 bytes which makes for a
simple subtract and shift, instead of a costly divide or reciprocol
multiply.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/include/asm/page_64.h

@@ -38,6 +38,8 @@
 
 #ifndef __ASSEMBLY__
 
+#define WANT_PAGE_VIRTUAL
+
 extern void _clear_page(void *page);
 #define clear_page(X)	_clear_page((void *)(X))
 struct page;