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.

Change-Id: I06f5e5820453d9df867f083118bf02b0ebd45ca9