sorting out the freeze / quiesce mess

[Christoph Hellwig]

Hi Jens and Ming,

I've been looking into properly supporting queue freezing for bio based
drivers (that is only release q_usage_counter on bio completion for them).
And the deeper I look into the code the more I'm confused by us having
the blk_mq_quiesce* interface in addition to blk_freeze_queue.  What
is a good reason to do a quiesce separately from a freeze?