igb: Don't receive packets when the napi budget == 0

Processing any incoming packets with a with a napi budget of 0
is incorrect driver behavior.

This matters as netpoll will shortly call drivers with a budget of 0
to avoid receive packet processing happening in hard irq context.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -6946,7 +6946,7 @@ static bool igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
 	unsigned int total_bytes = 0, total_packets = 0;
 	u16 cleaned_count = igb_desc_unused(rx_ring);
 
-	do {
+	while (likely(total_packets < budget)) {
 		union e1000_adv_rx_desc *rx_desc;
 
 		/* return some buffers to hardware, one at a time is too slow */
@@ -6998,7 +6998,7 @@ static bool igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
 
 		/* update budget accounting */
 		total_packets++;
-	} while (likely(total_packets < budget));
+	}
 
 	/* place incomplete frames back on ring for completion */
 	rx_ring->skb = skb;