[PATCH RFC] use WQ_MEM_RECLAIM for m_log_workqueue

[Chris Mason]

On Wed, Nov 04, 2015 at 01:51:03PM -0500, Chris Mason wrote:
> I think we should be using WQ_MEM_RECLAIM to make sure this thread pool
> makes progress when we're not able to allocate new workers.

Thinking harder, it's probably best to just flag them all
WQ_MEM_RECLAIM.  This is what btrfs does, and it saves you from painful
discoveries about how different queues depend on each other.

Tejun did verify in the dump that progress on m_log_workqueue was stuck
waiting for more threads.

I'll start testing and send a v2.

-chris