MINOR: dynbuf: provide a b_dequeue() variant for multi-thread

MINOR: dynbuf: provide a b_dequeue() variant for multi-thread

In order to forcefully unregister a buffer waiter during an inter-thread
takeover under isolation, we'll need to that the function works without
th_ctx but the target thread's ctx instead. Let's implement this by
passing the target thread as an argument. Now b_dequeue() simply calls
this one with tid. It's OK it's not on that critical a path, especially
since the list has been checked for existence before performing the call.