smp: Optimize flush_smp_call_function_queue()

The call_single_queue can contain (two) different callbacks,
synchronous and asynchronous. The current interrupt handler runs them
in-order, which means that remote CPUs that are waiting for their
synchronous call can be delayed by running asynchronous callbacks.

Rework the interrupt handler to first run the synchonous callbacks.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161907.836818381@infradead.org