selftests/xsk: Support tx_metadata_len

Add new config field and propagate to UMEM registration setsockopt.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231127190319.1190813-10-sdf@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>