m68k: rename __iounmap and mark it static

m68k uses __iounmap as the name for an internal helper that is only
used for some CPU types.  Mark it static, give it a better name
and move it around a bit to avoid a forward declaration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/kmap.h

@@ -20,7 +20,6 @@ extern void __iomem *__ioremap(unsigned long physaddr, unsigned long size,
 			       int cacheflag);
 #define iounmap iounmap
 extern void iounmap(void __iomem *addr);
-extern void __iounmap(void *addr, unsigned long size);
 
 #define ioremap ioremap
 static inline void __iomem *ioremap(unsigned long physaddr, unsigned long size)