net: Make RX-FCS and LRO mutually exclusive

LRO and RX-FCS offloads cannot be enabled at the same time since it is
not clear what should happen to the FCS of each coalesced packet.
The FCS is not really part of the TCP payload, hence cannot be merged
into one big packet. On the other hand, providing one big LRO packet
with one FCS contradicts the RX-FCS feature goal.

Use the fix features mechanism in order to prevent intersection of the
features and drop LRO in case RX-FCS is requested.

Enabling RX-FCS while LRO is enabled will result in:
$ ethtool -K ens6 rx-fcs on
Actual changes:
large-receive-offload: off [requested on]
rx-fcs: on
Signed-off-by: Gal Pressman <galp@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -7542,6 +7542,12 @@ static netdev_features_t netdev_fix_features(struct net_device *dev,
 		}
 	}
 
+	/* LRO feature cannot be combined with RX-FCS */
+	if ((features & NETIF_F_LRO) && (features & NETIF_F_RXFCS)) {
+		netdev_dbg(dev, "Dropping LRO feature since RX-FCS is requested.\n");
+		features &= ~NETIF_F_LRO;
+	}
+
 	return features;
 }