Heiko Carstens figured out that offlining a cpu can leak mm_structs because
the dying cpu's idle task fails to switch to init_mm and mmdrop its
active_mm before the cpu is down.  This patch introduces idle_task_exit,
which allows the idle task to do this as Ingo suggested.

I will follow this up with a patch for ppc64 which calls idle_task_exit
from cpu_die.
Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>