xsk: Require XDP_UMEM_TX_METADATA_LEN to actuate tx_metadata_len

Julian reports that commit 341ac980 ("xsk: Support tx_metadata_len")
can break existing use cases which don't zero-initialize xdp_umem_reg
padding. Introduce new XDP_UMEM_TX_METADATA_LEN to make sure we
interpret the padding as tx_metadata_len only when being explicitly
asked.

Fixes: 341ac980 ("xsk: Support tx_metadata_len")
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/20240713015253.121248-2-sdf@fomichev.me