um: Stop abusing __KERNEL__

commit 298e20ba8c197e8d429a6c8671550c41c7919033 upstream.

Currently UML is abusing __KERNEL__ to distinguish between
kernel and host code (os-Linux). It is better to use a custom
define such that existing users of __KERNEL__ don't get confused.

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Bernie Innocenti <codewiz@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>