ethdev: add Tx offload outer UDP checksum definition

ethdev: add Tx offload outer UDP checksum definition

Introduced DEV_TX_OFFLOAD_OUTER_UDP_CKSUM offload flags and
PKT_TX_OUTER_UDP_CKSUM mbuf ol_flags to enable Tx outer UDP
checksum offload.

To use hardware Tx outer UDP checksum offload, the user needs to,

- enable following in mbuf:
a) fill outer_l2_len and outer_l3_len in mbuf
b) set the PKT_TX_OUTER_UDP_CKSUM flag
c) set the flag PKT_TX_OUTER_IPV4 or PKT_TX_OUTER_IPV6

- configure DEV_TX_OFFLOAD_OUTER_UDP_CKSUM offload flags in slow path

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>