cxgbei: Add support for zero-copy iSCSI target transmission/read.

• Switch to allocating the cxgbei version of icl_pdu explicitly as a separate refcounted object allocated via malloc/free instead of storing it in the bhs mbuf prior to the bhs.

• Support the icl_conn_pdu_queue_cb() method to set a callback on a PDU to be invoked when the PDU is freed.

• For ICL_NOCOPY buffers, use an external mbuf to manage the storage for the buffer via m_extaddref(). Each external mbuf holds a reference on the associated PDU, so the callback is invoked once all of the external mbufs have been freed.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29910