cxgbe(4): Catch up with in-flight netmap rx before destroying queues.

The netmap application using the driver is responsible for replenishing
the receive freelists and they may be totally depleted when the
application exits. Packets in flight, if any, might block the pipeline
in case there aren't enough buffers left in the freelist. Avoid this by
filling up the freelists with a driver allocated buffer.

MFC after: 1 week
Sponsored by: Chelsio Communications