m68k: Allow ioremapping top of memory

The test in __ioremap to reject memory ranges crossing the 0 boundary
rejects also memory ranges ending at the end of the memory.  Fix that.
Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/mm/kmap.c

@@ -116,7 +116,7 @@ void __iomem *__ioremap(unsigned long physaddr, unsigned long size, int cachefla
 	/*
 	 * Don't allow mappings that wrap..
 	 */
-	if (!size || size > physaddr + size)
+	if (!size || physaddr > (unsigned long)(-size))
 		return NULL;
 
 #ifdef CONFIG_AMIGA