blk-mq-sched: add flush insertion into blk_mq_sched_insert_request()

Instead of letting the caller check this and handle the details
of inserting a flush request, put the logic in the scheduler
insertion function. This fixes direct flush insertion outside
of the usual make_request_fn calls, like from dm via
blk_insert_cloned_request().

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index f8de2db..54c8436 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -106,6 +106,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 	struct sbq_wait_state *ws;
 	DEFINE_WAIT(wait);
 	unsigned int tag_offset;
+	bool drop_ctx;
 	int tag;
 
 	if (data->flags & BLK_MQ_REQ_RESERVED) {
@@ -128,6 +129,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		return BLK_MQ_TAG_FAIL;
 
 	ws = bt_wait_ptr(bt, data->hctx);
+	drop_ctx = data->ctx == NULL;
 	do {
 		prepare_to_wait(&ws->wait, &wait, TASK_UNINTERRUPTIBLE);
 
@@ -150,7 +152,8 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		if (tag != -1)
 			break;
 
-		blk_mq_put_ctx(data->ctx);
+		if (data->ctx)
+			blk_mq_put_ctx(data->ctx);
 
 		io_schedule();
 
@@ -166,6 +169,9 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		ws = bt_wait_ptr(bt, data->hctx);
 	} while (1);
 
+	if (drop_ctx && data->ctx)
+		blk_mq_put_ctx(data->ctx);
+
 	finish_wait(&ws->wait, &wait);
 
 found_tag: