This patch adds support for XDP_TX action which enables XDP program to
transmit back received frames.

This patch has been tested with the "xdp2" app located in samples/bpf
dir. The DUT receives burst traffic packet generated using pktgen script
'pktgen_sample03_burst_single_flow.sh'.

v4: Moved stmmac_tx_timer_arm() to be done once at the end of NAPI RX.
    Fixed stmmac_xdp_xmit_back() to return STMMAC_XDP_CONSUMED if
    XDP buffer to frame conversion fails. Thanks to Jakub's input.

v3: Added 'nq->trans_start = jiffies' to avoid TX time-out as we are
    sharing TX queue between slow path and XDP. Thanks to Jakub Kicinski
    for pointing out.
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>