igb: Fix oops on changing number of rings

When changing the number of rings by ethtool -L, q_vectors are reused,
which causes oops because of uninitialized pointers.

- When an rx is reused as a tx, q_vector->rx.ring is not set to NULL, which
  misleads igb_poll() to determine that it has an rx ring although it
  actually points to the tx ring.
- When a tx is reused as an rx, q_vector->rx.ring->skb
  (q_vector->ring[0].skb) has a value that was used as tx_stats before.

Fix these problems by zeroing it out on reuseing it.

Fixes: 02ef6e1d0b ("igb: Fix queue allocation method to accommodate changing during runtime")
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

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

@@ -1207,6 +1207,8 @@ static int igb_alloc_q_vector(struct igb_adapter *adapter,
 	q_vector = adapter->q_vector[v_idx];
 	if (!q_vector)
 		q_vector = kzalloc(size, GFP_KERNEL);
+	else
+		memset(q_vector, 0, size);
 	if (!q_vector)
 		return -ENOMEM;