x86: make ZERO_PAGE() at least parse its argument

This doesn't really do anything, but at least we now parse teh
ZERO_PAGE() address argument so that we'll catch the most obvious errors
in usage next time they'll happen.

See commit 6a5c5d26c4 ("rdma: fix build errors on s390 and MIPS due to
bad ZERO_PAGE use") what happens when we don't have any use of the macro
argument at all.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/pgtable.h

@@ -46,7 +46,7 @@ void ptdump_walk_user_pgd_level_checkwx(void);
  */
 extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
 	__visible;
-#define ZERO_PAGE(vaddr) (virt_to_page(empty_zero_page))
+#define ZERO_PAGE(vaddr) ((void)(vaddr),virt_to_page(empty_zero_page))
 
 extern spinlock_t pgd_lock;
 extern struct list_head pgd_list;