stop_machine: Store task reference in a separate per cpu variable

To allow the stopper thread being managed by the smpboot
thread
infrastructure separate out the task storage from the stopper
data
structure.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc:
Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell
<rusty@rustcorp.com.au>
Cc: Paul McKenney
<paulmck@linux.vnet.ibm.com>
Cc: Srivatsa S. Bhat
<srivatsa.bhat@linux.vnet.ibm.com>
Cc: Arjan van de Veen
<arjan@infradead.org>
Cc: Paul Turner <pjt@google.com>
Cc: Richard
Weinberger <rw@linutronix.de>
Cc: Magnus Damm
<magnus.damm@gmail.com>
Link:
http://lkml.kernel.org/r/20130131120741.626690384@linutronix.de
Signed-off-by:
Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: franciscofranco
<franciscofranco.1990@gmail.com>

idea is taken here -
https://github.com/franciscofranco/M8/commit/0e0b9c82b72cfe016416559018b3310fcd1a3022