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: Brian King <brking@linux.vnet.ibm.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>