Add dedicated transmit function and transmit completion handler for
XDP.  The XDP transmit logic and completion logic are different than
regular TX ring.  The TX buffer is recycled back to the RX ring when
it completes.

v3: Improved the buffer recyling scheme for XDP_TX.

v2: Add trace_xdp_exception().
    Add dma_sync.
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Tested-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>