net: mctp: unconditionally set skb->dev on dst output

On transmit, we are currently relying on skb->dev being set by
mctp_local_output() when we first set up the skb destination fields.
However, forwarded skbs do not use the local_output path, so will retain
their incoming netdev as their ->dev on tx. This does not work when
we're forwarding between interfaces.

Set skb->dev unconditionally in the transmit path, to allow for proper
forwarding.

We keep the skb->dev initialisation in mctp_local_output(), as we use it
for fragmentation.

Fixes: 269936d ("net: mctp: separate routing database from routing operations")
Suggested-by: Vince Chang <vince_chang@aspeedtech.com>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20251125-dev-forward-v1-1-54ecffcd0616@codeconstruct.com.au
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: jk-ozlabs

net/mctp/route.c

@@ -623,6 +623,7 @@ static int mctp_dst_output(struct mctp_dst *dst, struct sk_buff *skb)
 
 	skb->protocol = htons(ETH_P_MCTP);
 	skb->pkt_type = PACKET_OUTGOING;
+	skb->dev = dst->dev->dev;
 
 	if (skb->len > dst->mtu) {
 		kfree_skb(skb);