um: return error from ioremap()

Back a few years ago, ioremap() was added to UML so that we'd
not break the build for everything all the time. However, for
some reason, v1 of the patch got applied, rather than the v2
that returned NULL, which was discussed here:

https://lore.kernel.org/lkml/1495726955-27497-1-git-send-email-logang@deltatee.com/

Fix that now.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/asm/io.h

@@ -5,7 +5,7 @@
 #define ioremap ioremap
 static inline void __iomem *ioremap(phys_addr_t offset, size_t size)
 {
-	return (void __iomem *)(unsigned long)offset;
+	return NULL;
 }
 
 #define iounmap iounmap