Patch from Dipankar Sarma  <dipankar@in.ibm.com>

This is Manfred's patch which provides a CPU_UP_PREPARE cpu notifier to
allow initialization of per_cpu data just before the cpu becomes fully
functional.

It also provides a facility for the CPU_UP_PREPARE handler to return
NOTIFY_BAD to signify that the CPU is not permitted to come up.  If
that happens, a CPU_UP_CANCELLED message is passed to all the handlers.

The patch also fixes a bogus NOFITY_BAD return from the softirq setup
code.

Patch has been acked by Rusty.

We need this mechanism in slab for starting per-cpu timers and for
allocating the per-cpu slab hgead arrays *before* the CPU has come up
and started using slab.