net: dsa: don't error out when drivers return ETH_DATA_LEN in .port_max_mtu()

Currently, when dsa_slave_change_mtu() is called on a user port where
dev->max_mtu is 1500 (as returned by ds->ops->port_max_mtu()), the code
will stumble upon this check:

	if (new_master_mtu > mtu_limit)
		return -ERANGE;

because new_master_mtu is adjusted for the tagger overhead but mtu_limit
is not.

But it would be good if the logic went through, for example if the DSA
master really depends on an MTU adjustment to accept DSA-tagged frames.

To make the code pass through the check, we need to adjust mtu_limit for
the overhead as well, if the minimum restriction was caused by the DSA
user port's MTU (dev->max_mtu). A DSA user port MTU and a DSA master MTU
are always offset by the protocol overhead.

Currently no drivers return 1500 .port_max_mtu(), but this is only
temporary and a bug in itself - mv88e6xxx should have done that, but
since commit b9c587fe ("dsa: mv88e6xxx: Include tagger overhead when
setting MTU for DSA and CPU ports") it no longer does. This is a
preparation for fixing that.

Fixes: bfcb8132 ("net: dsa: configure the MTU for switch ports")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -1933,6 +1933,7 @@ int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
 	int new_master_mtu;
 	int old_master_mtu;
 	int mtu_limit;
+	int overhead;
 	int cpu_mtu;
 	int err;
 
@@ -1961,9 +1962,10 @@ int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
 			largest_mtu = slave_mtu;
 	}
 
-	mtu_limit = min_t(int, master->max_mtu, dev->max_mtu);
+	overhead = dsa_tag_protocol_overhead(cpu_dp->tag_ops);
+	mtu_limit = min_t(int, master->max_mtu, dev->max_mtu + overhead);
 	old_master_mtu = master->mtu;
-	new_master_mtu = largest_mtu + dsa_tag_protocol_overhead(cpu_dp->tag_ops);
+	new_master_mtu = largest_mtu + overhead;
 	if (new_master_mtu > mtu_limit)
 		return -ERANGE;
 
@@ -1998,8 +2000,7 @@ int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
 
 out_port_failed:
 	if (new_master_mtu != old_master_mtu)
-		dsa_port_mtu_change(cpu_dp, old_master_mtu -
-				    dsa_tag_protocol_overhead(cpu_dp->tag_ops));
+		dsa_port_mtu_change(cpu_dp, old_master_mtu - overhead);
 out_cpu_failed:
 	if (new_master_mtu != old_master_mtu)
 		dev_set_mtu(master, old_master_mtu);