selftests/bpf: Add XDP_UMEM_TX_METADATA_LEN to XSK TX metadata test

This flag is now required to use tx_metadata_len.

Fixes: 40808a23

 ("selftests/bpf: Add TX side to xdp_metadata")
Reported-by: Julian Schindel <mail@arctic-alpaca.de>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20240713015253.121248-3-sdf@fomichev.me