Fix race between attach_task and cpuset_exit

Currently cpuset_exit() changes the exiting task's ->cpuset pointer w/o
taking task_lock().  This can lead to ugly races between attach_task and
cpuset_exit.  Details of the races are described at
http://lkml.org/lkml/2007/3/24/132

.

Patch below closes those races.

Signed-off-by: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>