cxgb4: add UDP segmentation offload support

Implement and export UDP segmentation offload (USO) support for both
NIC and MQPRIO QoS offload Tx path. Update appropriate logic in Tx to
parse GSO info in skb and configure FW_ETH_TX_EO_WR request needed to
perform USO.

v2:
- Remove inline keyword from write_eo_udp_wr() in sge.c. Let the
  compiler decide.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>