MINOR: dynbuf: provide a b_dequeue() function to detach a bw from the queue

MINOR: dynbuf: provide a b_dequeue() function to detach a bw from the queue

Now that we need to keep the bitmap in sync with the list heads, we don't
want tasks to leave just doing a LIST_DEL_INIT() without updating the map.
Let's provide a b_dequeue() function for that purpose. The function detects
when it's going to remove the last element and figures the queue number
based on the pointer since it points to the root. It's not used yet.