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 cc5b114dcf98 ("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 924708081629 ("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: cc5b114dcf98 ("bpf, i40e: add meta data support")
Fixes: 0c8493d90b6b ("i40e: add XDP support for pass and drop actions")
Reported-by: Keith Busch <[email protected]>
Reported-by: Toshiaki Makita <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Cc: Björn Töpel <[email protected]>
Cc: John Fastabend <[email protected]>
Tested-by: Keith Busch <[email protected]>
Acked-by: John Fastabend <[email protected]>
Acked-by: Alexander Duyck <[email protected]>
Signed-off-by: Jeff Kirsher <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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