sched: Simplify finding the lowest sched_domain

Instead of relying on knowing the build order and various CONFIG_
flags simply remember the bottom most sched_domain when we created the
domain hierarchy.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110407122942.134511046@chello.nl


Signed-off-by: Ingo Molnar <mingo@elte.hu>