[RFC] don't create cpu/online sysfs file

[Dave Hansen]

I sent this out earlier this week, but wanted to make sure everybody is
happy with it before I sent it upstream.  I'm doing it backwards:
[PATCH] before [RFC]. :)

Rusty said he didn't have a problem with it already.  Anton, do mind
putting this in your patch queue, or should I send it upstream myself?

--

The following patch restricts creation of the
/sys/devices/system/cpu/cpuX/online file to conditions where it is known
that cpu hotplugging might be possible.  It also leaves room for a much
more complex method of deciding this information in the future, on a
per-cpu basis.

It fixes a bug where CPU hotplug ppc64 RTAS calls were being made on a
non-partitioned system.  This caused RTAS assertions, which are bad.

Any new architecture that defines CONFIG_HOTPLUG_CPU will need to
implement a cpu_is_hotpluggable() function.  This could be overcome by a
series of #ifdefs, but I think someone porting CPU hotplug to a new
architecture can figure this one out.
                                                                                                                   Tested on 16-way ppc64 LPAR and 8-way bare-metal ppc64.  Compile tested
on x86.

-- Dave
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