Move BUG_ONs to include/bug.h

We will need futexes to use in PIE code but futex.h
uses BUG_ON helper, so to diet inclusions move BUG_ONs
code to include/bug.h.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>