workqueue: reimplement work_on_cpu() using system_wq

The existing work_on_cpu() implementation is hugely inefficient.  It
creates a new kthread, execute that single function and then let the
kthread die on each invocation.

Now that system_wq can handle concurrent executions, there's no
advantage of doing this.  Reimplement work_on_cpu() using system_wq
which makes it simpler and way more efficient.

stable: While this isn't a fix in itself, it's needed to fix a
        workqueue related bug in cpufreq/powernow-k8.  AFAICS, this
        shouldn't break other existing users.

Signed-off-by: Tejun Heo <[email protected]>
Acked-by: Jiri Kosina <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: [email protected]

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