x86: processor.h - use PAGE_SIZE instead of numeric value

This patch replaces numeric constant with an appropriate macro

Also 0x800000000000UL is changed to bit shifting which is complement
to the code comment (thanks hpa for notice)
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/processor.h

@@ -870,7 +870,7 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
 /*
  * User space process size. 47bits minus one guard page.
  */
-#define TASK_SIZE64		(0x800000000000UL - 4096)
+#define TASK_SIZE64	((1UL << 47) - PAGE_SIZE)
 
 /* This decides where the kernel will search for a free chunk of vm
  * space during mmap's.