um: Stop abusing __KERNEL__

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.

Change-Id: I37a472e7405405c5da6b23bd3c279e4a0cb76cd0
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 298e20ba8c197e8d429a6c8671550c41c7919033
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Srinivasarao P <spathi@codeaurora.org>