lib/percpu_counter.c: fix bad percpu counter state during suspend

I got a bug report yesterday from Laszlo Ersek in which he states that
his kvm instance fails to suspend.  Laszlo bisected it down to this
commit 1cf7e9c6 ("virtio_blk: blk-mq support") where virtio-blk is
converted to use the blk-mq infrastructure.

After digging a bit, it became clear that the issue was with the queue
drain.  blk-mq tracks queue usage in a percpu counter, which is
incremented on request alloc and decremented when the request is freed.
The initial hunt was for an inconsistency in blk-mq, but everything
seemed fine.  In fact, the counter only returned crazy values when
suspend was in progress.

When a CPU is unplugged, the percpu counters merges that CPU state with
the general state.  blk-mq takes care to register a hotcpu notifier with
the appropriate priority, so we know it runs after the percpu counter
notifier.  However, the percpu counter notifier only merges the state
when the CPU is fully gone.  This leaves a sta...