[PATCH] cpuset: oops in exit on null cpuset fix

Fix a latent bug in cpuset_exit() handling.  If a task tried to allocate
memory after calling cpuset_exit(), it oops'd in
cpuset_update_task_memory_state() on a NULL cpuset pointer.

So set the exiting tasks cpuset to the root cpuset instead of to NULL.

A distro kernel hit this with an added kernel package that had just such a
hook (allocating memory) in the exit code path.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>