powerpc/pmac: replace current->state by set_current_state()

Use helper functions to access current->state.
Direct assignments are prone to races and therefore buggy.

current->state = TASK_RUNNING can be replaced by __set_current_state()

Thanks to Peter Zijlstra for the exact definition of the problem.

Suggested-By: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>