block: move queue run on unplug to kblockd

There are worries that we are now consuming a lot more stack in
some cases, since we potentially call into IO dispatch from
schedule() or io_schedule(). We can reduce this problem by moving
the running of the queue to kblockd, like the old plugging scheme
did as well.

This may or may not be a good idea from a performance perspective,
depending on how many tasks have queue plugs running at the same
time. For even the slightly contended case, doing just a single
queue run from kblockd instead of multiple runs directly from the
unpluggers will be faster.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-core.c

@@ -2665,7 +2665,7 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
 static void queue_unplugged(struct request_queue *q, unsigned int depth)
 {
 	trace_block_unplug_io(q, depth);
-	__blk_run_queue(q, false);
+	__blk_run_queue(q, true);
 
 	if (q->unplugged_fn)
 		q->unplugged_fn(q);