selftests/bpf: Actuate tx_metadata_len in xdp_hw_metadata

set XDP_UMEM_TX_METADATA_LEN flag to reserve tx_metadata_len bytes of
per-chunk metadata.

Fixes: d5e726d9 ("xsk: Require XDP_UMEM_TX_METADATA_LEN to actuate tx_metadata_len")
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/20241205044258.3155799-1-yoong.siang.song@intel.com