i40e/i40evf: Moves skb from i40e_rx_buffer to i40e_ring

i40e/i40evf: Moves skb from i40e_rx_buffer to i40e_ring

This patch reduces the size of struct i40e_rx_buffer by one pointer,
and makes the i40e driver a little more consistent with the igb driver
in terms of packets that span buffers.

We do this by moving the skb field from struct i40e_rx_buffer to
struct i40e_ring. We pass the skb we already have (or NULL if we
don't) to i40e_fetch_rx_buffer(), which skips the skb allocation if we
already have one for this packet.

Change-ID: I4ad48a531844494ba0c5d8e1a62209a057f661b0
Signed-off-by: Scott Peterson <scott.d.peterson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Orabug: 26132523
(cherry picked from commit e72e56597ba15ce70f4fc1eb2ceeaa8da0d4ab5e)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Brian Maly <brian.maly@oracle.com>