block: fix request_queue->flags initialization

A queue newly allocated with blk_alloc_queue_node() has only QUEUE_FLAG_BYPASS set.  For request-based drivers, blk_init_allocated_queue() is called and q->queue_flags is overwritten with QUEUE_FLAG_DEFAULT which doesn't include BYPASS even though the initial bypass is still in effect.

In blk_init_allocated_queue(), or QUEUE_FLAG_DEFAULT to q->queue_flags instead of overwriting.