i40e/i40evf: Use build_skb to build frames

i40e/i40evf: Use build_skb to build frames

This patch is meant to improve the performance of the Rx path.
Specifically by using build_skb we have several distinct advantages.

In the case of small frames we were previously using a copy-break approach.
This means that we were allocating a page fragment to use for skb->head,
and were having to copy the packet into that region.  Both of those calls
are now avoided since we just build the skb around the data.

In the case of large frames the gains are much more significant.
Specifically we were having to allocate skb->head, and copy the headers as
before.  However in addition we were having to parse the header using
eth_get_headlen which could be quite expensive.  All of this is avoided by
building the frame around the data.  I have seen gains as high as 30% when
using VXLAN for instance due to just header pulling overhead.

Finally with all this in place it also sets us up to start looking at
enabling XDP.  Specifically we now have a path in which the data is in the
page and the frame is built around it.  So if we parse it with XDP before
we call build_skb we can take care of any necessary processing there.

Change-ID: Id4bdd618e94473d41f892417e5d8019639e421e3
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>