i40e/i40evf: Add support for IPv4 encapsulated in IPv6

This patch fixes two issues.  First was the fact that iphdr(skb)->protocl
was being used to test for the outer transport protocol.  This completely
breaks IPv6 support.  Second was the fact that we cleared the flag for v4
going to v6, but we didn't take care of txflags going the other way.  As
such we would have the v6 flag still set even if the inner header was v4.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>