bpf, xdp, i40e: fix i40e_build_skb skb reserve and truesize

Using skb_reserve(skb, I40E_SKB_PAD + (xdp->data - xdp->data_hard_start))
is clearly wrong since I40E_SKB_PAD already points to the offset where
the original xdp->data was sitting since xdp->data_hard_start is defined
as xdp->data - i40e_rx_offset(rx_ring) where latter offsets to I40E_SKB_PAD
when build skb is used.

However, also before cc5b114d ("bpf, i40e: add meta data support")
this seems broken since bpf_xdp_adjust_head() helper could have been used
to alter headroom and enlarge / shrink the frame and with that the assumption
that the xdp->data remains unchanged does not hold and would push a bogus
packet to upper stack.

ixgbe got this right in 92470808 ("ixgbe: add XDP support for pass and
drop actions"). In any case, fix it by removing the I40E_SKB_PAD from both
skb_reserve() and truesize calculation.

Fixes: cc5b114d ("bpf, i40e: add meta data support")
Fixes: 0c8493d9 ("i40e: add XDP support for pass and drop actions")
Reported-by: Keith Busch <keith.busch@linux.intel.com>
Reported-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Björn Töpel <bjorn.topel@intel.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Tested-by: Keith Busch <keith.busch@linux.intel.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>