selftests/bpf: Enable Tx hwtstamp in xdp_hw_metadata

Currently, user needs to manually enable transmit hardware timestamp
feature of certain Ethernet drivers, e.g. stmmac and igc drivers, through
following command after running the xdp_hw_metadata app.

sudo hwstamp_ctl -i eth0 -t 1

To simplify the step test of xdp_hw_metadata, set tx_type to HWTSTAMP_TX_ON
to enable hardware timestamping for all outgoing packets, so that user no
longer need to execute hwstamp_ctl command.

Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241205051936.3156307-1-yoong.siang.song@intel.com