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: 48eb03d ("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

include/net/xdp_sock.h

@@ -188,6 +188,8 @@ static inline void xsk_tx_metadata_complete(struct xsk_tx_metadata_compl *compl,
 {
 	if (!compl)
 		return;
+	if (!compl->tx_timestamp)
+		return;
 
 	*compl->tx_timestamp = ops->tmo_fill_timestamp(priv);
 }