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>