via-velocity: fix netif_receive_skb use in irq disabled section.

2fdac010bd ("via-velocity.c: update napi
implementation") overlooked an irq disabling spinlock when the Rx part
of the NAPI poll handler was converted from netif_rx to netif_receive_skb.

NAPI Rx processing can be taken out of the locked section with a pair of
napi_{disable / enable} since it only races with the MTU change function.

An heavier rework of the NAPI locking would be able to perform NAPI Tx
before Rx where I simply removed one of velocity_tx_srv calls.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1022733
Fixes: 2fdac010bd (via-velocity.c: update napi implementation)
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Alex A. Schmidt <aaschmidt1@gmail.com>
Cc: Jamie Heilman <jamie@audible.transient.net>
Cc: Michele Baldessari <michele@acksyn.org>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/via/via-velocity.c

@@ -2172,16 +2172,13 @@ static int velocity_poll(struct napi_struct *napi, int budget)
 	unsigned int rx_done;
 	unsigned long flags;
 
-	spin_lock_irqsave(&vptr->lock, flags);
 	/*
 	 * Do rx and tx twice for performance (taken from the VIA
 	 * out-of-tree driver).
 	 */
-	rx_done = velocity_rx_srv(vptr, budget / 2);
+	rx_done = velocity_rx_srv(vptr, budget);
+	spin_lock_irqsave(&vptr->lock, flags);
 	velocity_tx_srv(vptr);
-	rx_done += velocity_rx_srv(vptr, budget - rx_done);
-	velocity_tx_srv(vptr);
-
 	/* If budget not fully consumed, exit the polling mode */
 	if (rx_done < budget) {
 		napi_complete(napi);
@@ -2342,6 +2339,8 @@ static int velocity_change_mtu(struct net_device *dev, int new_mtu)
 		if (ret < 0)
 			goto out_free_tmp_vptr_1;
 
+		napi_disable(&vptr->napi);
+
 		spin_lock_irqsave(&vptr->lock, flags);
 
 		netif_stop_queue(dev);
@@ -2362,6 +2361,8 @@ static int velocity_change_mtu(struct net_device *dev, int new_mtu)
 
 		velocity_give_many_rx_descs(vptr);
 
+		napi_enable(&vptr->napi);
+
 		mac_enable_int(vptr->mac_regs);
 		netif_start_queue(dev);