xsk: Don't assume metadata is always requested in TX completion

`compl->tx_timestam != NULL` means that the user has explicitly
requested the metadata via XDP_TX_METADATA+XDP_TX_METADATA_TIMESTAMP.

Fixes: 48eb03dd ("xsk: Add TX timestamp and TX checksum offload support")
Reported-by: Daniele Salvatore Albano <d.albano@gmail.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Daniele Salvatore Albano <d.albano@gmail.com>
Link: https://lore.kernel.org/bpf/20240318165427.1403313-1-sdf@google.com