iavf: fix netdev->max_mtu to respect actual hardware limit

iavf sets LIBIE_MAX_MTU as netdev->max_mtu, ignoring vf_res->max_mtu
from PF [1]. This allows setting an MTU beyond the actual hardware
limit, causing TX queue timeouts [2].

Set correct netdev->max_mtu using vf_res->max_mtu from the PF.

Note that currently PF drivers such as ice/i40e set the frame size in
vf_res->max_mtu, not MTU. Convert vf_res->max_mtu to MTU before setting
netdev->max_mtu.

[1]
 # ip -j -d link show $DEV | jq '.[0].max_mtu'
 16356

[2]
 iavf 0000:00:05.0 enp0s5: NETDEV WATCHDOG: CPU: 1: transmit queue 0 timed out 5692 ms
 iavf 0000:00:05.0 enp0s5: NIC Link is Up Speed is 10 Gbps Full Duplex
 iavf 0000:00:05.0 enp0s5: NETDEV WATCHDOG: CPU: 6: transmit queue 3 timed out 5312 ms
 iavf 0000:00:05.0 enp0s5: NIC Link is Up Speed is 10 Gbps Full Duplex
 ...

Fixes: 5fa4caf ("iavf: switch to Page Pool")
Signed-off-by: Kohei Enju <kohei@enjuk.jp>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Author: hyperenju

drivers/net/ethernet/intel/iavf/iavf_main.c

@@ -2793,7 +2793,22 @@ static void iavf_init_config_adapter(struct iavf_adapter *adapter)
 	netdev->watchdog_timeo = 5 * HZ;
 
 	netdev->min_mtu = ETH_MIN_MTU;
-	netdev->max_mtu = LIBIE_MAX_MTU;
+
+	/* PF/VF API: vf_res->max_mtu is max frame size (not MTU).
+	 * Convert to MTU.
+	 */
+	if (!adapter->vf_res->max_mtu) {
+		netdev->max_mtu = LIBIE_MAX_MTU;
+	} else if (adapter->vf_res->max_mtu < LIBETH_RX_LL_LEN + ETH_MIN_MTU ||
+		   adapter->vf_res->max_mtu >
+			   LIBETH_RX_LL_LEN + LIBIE_MAX_MTU) {
+		netdev_warn_once(adapter->netdev,
+				 "invalid max frame size %d from PF, using default MTU %d",
+				 adapter->vf_res->max_mtu, LIBIE_MAX_MTU);
+		netdev->max_mtu = LIBIE_MAX_MTU;
+	} else {
+		netdev->max_mtu = adapter->vf_res->max_mtu - LIBETH_RX_LL_LEN;
+	}
 
 	if (!is_valid_ether_addr(adapter->hw.mac.addr)) {
 		dev_info(&pdev->dev, "Invalid MAC address %pM, using random\n",