block: change the tag sync vs async restriction logic

Make them fully share the tag space, but disallow async requests using
the last any two slots.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/blk-core.c b/block/blk-core.c
index 4906507..1c74840 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1815,7 +1815,7 @@
 	 * the driver side.
 	 */
 	if (blk_account_rq(rq))
-		q->in_flight++;
+		q->in_flight[rq_is_sync(rq)]++;
 }
 
 /**