null_blk: prevent timer handler running on a different CPU where started
When irqmode=2 (IRQ completion handler is timer), timer handler should
be called on the same CPU where the timer has been started.
Since completion_queues are per-cpu and the completion handler only
touches completion_queue for local CPU, we need to prevent the handler
from running on a different CPU where the timer has been started.
Otherwise, the IO cannot be completed until another completion handler
is executed on that CPU.