This fixes a use-after-free bug in the context switching.  A process going
out of context after exiting wakes up the next process and then kills
itself.  The problem is that when it gets around to killing itself is up to
the host and can happen a long time later, including after the incoming
process has freed its stack, and that memory is possibly being used for
something else.

The fix is to have the incoming process kill the exiting process just to
make sure it can't be running at the point that its stack is freed.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>