smp: Move irq_work_run() out of flush_smp_call_function_queue()

This ensures flush_smp_call_function_queue() is strictly about
call_single_queue.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161907.895109676@infradead.org