wireguard: send: append trailer after expanding head

wireguard: send: append trailer after expanding head

With how this is currently written, we add the trailer, zero it out, and
then add the header space on. If that header space requires a
reallocation + copy, the zeros in the trailer aren't copied, because the
skb len hasn't actually been yet expanded to cover that. Instead add the
padding at the end of the process rather than at the beginning.

Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20260529173134.3080773-2-Jason@zx2c4.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>