ixgbe: Fix skb list corruption on Power systems

This patch fixes an issue seen on Power systems with ixgbe which results
in skb list corruption and an eventual kernel oops. The following is what
was observed:

CPU 1                                   CPU2
============================            ============================
1: ixgbe_xmit_frame_ring                ixgbe_clean_tx_irq
2:  first->skb = skb                     eop_desc = tx_buffer->next_to_watch
3:  ixgbe_tx_map                         read_barrier_depends()
4:   wmb                                 check adapter written status bit
5:   first->next_to_watch = tx_desc      napi_consume_skb(tx_buffer->skb ..);
6:   writel(i, tx_ring->tail);

The read_barrier_depends is insufficient to ensure that tx_buffer->skb does not
get loaded prior to tx_buffer->next_to_watch, which then results in loading
a stale skb pointer. This patch replaces the read_barrier_depends with
smp_rmb to ensure loads are ordered with respect to the load of
tx_buffer->next_to_watch.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: NBrian King <brking@linux.vnet.ibm.com>
Acked-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -1192,7 +1192,7 @@ static bool ixgbe_clean_tx_irq(struct ixgbe_q_vector *q_vector,
 			break;
 
 		/* prevent any other reads prior to eop_desc */
-		read_barrier_depends();
+		smp_rmb();
 
 		/* if DD is not set pending work has not been completed */
 		if (!(eop_desc->wb.status & cpu_to_le32(IXGBE_TXD_STAT_DD)))