Buffers may be allocated for indirect descriptors by tx thread and are freed when tx thread is invoked in the next invocation. This is to allow the recipient (kernel) to have a chance to process them. But if the tap interface is deleted, the tx thread may not be called to clean up the indirect descriptors' buffers. We need to remove them without waiting for the tx thread to be called. Failure to do so may cause buffers leak when the tap interface is deleted.