smp: Provide generic idle thread allocation

All SMP architectures have magic to fork the idle task and to store it
for reusage when cpu hotplug is enabled. Provide a generic
infrastructure for it.

Create/reinit the idle thread for the cpu which is brought up in the
generic code and hand the thread pointer to the architecture code via
__cpu_up().

Note, that fork_idle() is called via a workqueue, because this
guarantees that the idle thread does not get a reference to a user
space VM. This can happen when the boot process did not bring up all
possible cpus and a later cpu_up() is initiated via the sysfs
interface. In that case fork_idle() would be called in the context of
the user space task and take a reference on the user space VM.

Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Srivatsa S. Bhat <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Russell King <[email protected]>
Cc: Mike Frysinger <[email protected]>
Cc: Jesper Nilsson <[email protected]>
Cc: Richard Kuo <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Hirokazu Takata <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: David Howells <[email protected]>
Cc: James E.J. Bottomley <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Paul Mundt <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: [email protected]
Acked-by: Venkatesh Pallipadi <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]

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