macsec: Enable devices to advertise whether they update sk_buff md_dst during offloads

Cannot know whether a Rx skb missing md_dst is intended for MACsec or not
without knowing whether the device is able to update this field during an
offload. Assume that an offload to a MACsec device cannot support updating
md_dst by default. Capable devices can advertise that they do indicate that
an skb is related to a MACsec offloaded packet using the md_dst.

Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: stable@vger.kernel.org
Fixes: 860ead89 ("net/macsec: Add MACsec skb_metadata_dst Rx Data path support")
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/20240423181319.115860-2-rrameshbabu@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/macsec.h

@@ -321,6 +321,7 @@ struct macsec_context {
  *	for the TX tag
  * @needed_tailroom: number of bytes reserved at the end of the sk_buff for the
  *	TX tag
+ * @rx_uses_md_dst: whether MACsec device offload supports sk_buff md_dst
  */
 struct macsec_ops {
 	/* Device wide */
@@ -352,6 +353,7 @@ struct macsec_ops {
 				 struct sk_buff *skb);
 	unsigned int needed_headroom;
 	unsigned int needed_tailroom;
+	bool rx_uses_md_dst;
 };
 
 void macsec_pn_wrapped(struct macsec_secy *secy, struct macsec_tx_sa *tx_sa);