powerpc/paravirt: Improve vcpu_is_preempted

PowerVM Hypervisor dispatches on a whole core basis. In a shared LPAR, a
CPU from a core that is CEDED or preempted may have a larger latency. In
such a scenario, its preferable to choose a different CPU to run.

If one of the CPUs in the core is active, i.e neither CEDED nor
preempted, then consider this CPU as not preempted.

Also if any of the CPUs in the core has yielded but OS has not requested
CEDE or CONFER, then consider this CPU to be preempted.

Correct detection of preempted CPUs is important for detecting idle
CPUs/cores in task scheduler.

Tested-by: Aboorva Devarajan <[email protected]>
Reviewed-by: Shrikanth Hegde <[email protected]>
Signed-off-by: Srikar Dronamraju <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

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