mm: check the argument of kunmap on architectures without highmem

If you're using a non-highmem architecture, passing an argument with the
wrong type to kunmap() doesn't give you a warning because the ifdef
doesn't check the type.

Using a static inline function solves the problem nicely.
Reported-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/highmem.h

@@ -55,7 +55,9 @@ static inline void *kmap(struct page *page)
 	return page_address(page);
 }
 
-#define kunmap(page) do { (void) (page); } while (0)
+static inline void kunmap(struct page *page)
+{
+}
 
 static inline void *kmap_atomic(struct page *page, enum km_type idx)
 {