net/sched: sch_cake: fix NAT destination port not being updated in cake_update_flowkeys

cake_update_flowkeys() is supposed to update the flow dissector keys
with the NAT-translated addresses and ports from conntrack, so that
CAKE's per-flow fairness correctly identifies post-NAT flows as
belonging to the same connection.

For the source port, this works correctly:
keys->ports.src = port;

But for the destination port, the assignment is reversed:
port = keys->ports.dst;

This means the NAT destination port is never updated in the flow keys.
As a result, when multiple connections are NATed to the same destination,
CAKE treats them as separate flows because the original (pre-NAT)
destination ports differ. This breaks CAKE's NAT-aware flow isolation
when using the "nat" mode.

The bug was introduced in commit b0c19ed6088a ("sch_cake: Take advantage
of skb->hash where appropriate") which refactored the original direct
assignment into a compare-and-conditionally-update pattern, but wrote
the destination port update backwards.

Fix by reversing the assignment direction to match the source port
pattern.

Fixes: b0c19ed6088a ("sch_cake: Take advantage of skb->hash where appropriate")
Signed-off-by: Dudu Lu <phx0fer@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20260413110041.44704-1-phx0fer@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>