Fix for a race condition when a task gets preempted by another task while
executing the destroy_context(...) in a FEW_CONTEXTS environment.
mm->context == NO_CONTEXT but the context_map may indicate all contexts are
in use.

The solution to this problem is to disable kernel preemption while
destroying a MMU context.
Signed-off-by: Guillaume Autran <gautran@mrv.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>