net: ionic: map SKB after pseudo-header checksum prep

The TSO path called ionic_tx_map_skb() before preparing the TCP pseudo
checksum (ionic_tx_tcp_[inner_]pseudo_csum()), which may perform
skb_cow_head() and might modifies bytes in the linear header area.

Mapping first and then mutating the header risks:
- Using a stale DMA address if skb_cow_head() relocates the head, and/or
- Device reading stale header bytes on weakly-ordered systems
(CPU writes after mapping are not guaranteed visible without an
explicit dma_sync_single_for_device()).

Reorder the TX path to perform all header mutations (including
skb_cow_head()) *before* DMA mapping. Mapping is now done only after the
skb layout and header contents are final. This removes the need for any
post-mapping dma_sync and prevents on-wire corruption observed under
VLAN+TSO load after repeated runs.

This change is purely an ordering fix; no functional behavior change
otherwise.

Fixes: 0f3154e6bcb3 ("ionic: Add Tx and Rx handling")
Signed-off-by: Mohammad Heib <[e-mail]>
Reviewed-by: Brett Creeley <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/pensando/ionic/ionic_txrx.c

···
 	bool encap;
 	int err;
 
-	desc_info = &q->tx_info[q->head_idx];
-
-	if (unlikely(ionic_tx_map_skb(q, skb, desc_info)))
-		return -EIO;
-
-	len = skb->len;
-	mss = skb_shinfo(skb)->gso_size;
-	outer_csum = (skb_shinfo(skb)->gso_type & (SKB_GSO_GRE |
-						   SKB_GSO_GRE_CSUM |
-						   SKB_GSO_IPXIP4 |
-						   SKB_GSO_IPXIP6 |
-						   SKB_GSO_UDP_TUNNEL |
-						   SKB_GSO_UDP_TUNNEL_CSUM));
 	has_vlan = !!skb_vlan_tag_present(skb);
 	vlan_tci = skb_vlan_tag_get(skb);
 	encap = skb->encapsulation;
···
 		err = ionic_tx_tcp_inner_pseudo_csum(skb);
 	else
 		err = ionic_tx_tcp_pseudo_csum(skb);
-	if (unlikely(err)) {
-		/* clean up mapping from ionic_tx_map_skb */
-		ionic_tx_desc_unmap_bufs(q, desc_info);
+	if (unlikely(err))
 		return err;
-	}
 
+	desc_info = &q->tx_info[q->head_idx];
+	if (unlikely(ionic_tx_map_skb(q, skb, desc_info)))
+		return -EIO;
+
+	len = skb->len;
+	mss = skb_shinfo(skb)->gso_size;
+	outer_csum = (skb_shinfo(skb)->gso_type & (SKB_GSO_GRE |
+						   SKB_GSO_GRE_CSUM |
+						   SKB_GSO_IPXIP4 |
+						   SKB_GSO_IPXIP6 |
+						   SKB_GSO_UDP_TUNNEL |
+						   SKB_GSO_UDP_TUNNEL_CSUM));
 	if (encap)
 		hdrlen = skb_inner_tcp_all_headers(skb);
 	else