block: queue lock must be acquired when iterating over rls

blk_set_queue_dying() does not acquire queue lock before it calls
blk_queue_for_each_rl(). This allows a racing blkg_destroy() to
remove blkg->q_node from the linked list and have
blk_queue_for_each_rl() loop infitely over the removed blkg->q_node
list node.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>