powerpc: check_and_cede_processor() never cedes

powerpc: check_and_cede_processor() never cedes

Commit f948501b36c6 ("Make hard_irq_disable() actually hard-disable
interrupts") caused check_and_cede_processor to stop working.
->irq_happened will never be zero right after a hard_irq_disable
so the compiler removes the call to cede_processor completely.

The bug was introduced back in the lazy interrupt handling rework
of 3.4 but was hidden until recently because hard_irq_disable did
nothing.

This issue will eventually appear in 3.4 stable since the
hard_irq_disable fix is marked stable, so mark this one for stable
too.

Signed-off-by: Anton Blanchard <[email protected]>
Cc: [email protected]
Signed-off-by: Benjamin Herrenschmidt <[email protected]>