net: libwx: fix Tx descriptor content for some tunnel packets

net: libwx: fix Tx descriptor content for some tunnel packets

The length of skb header was incorrectly calculated when transmit a tunnel
packet with outer IPv6 extension header, or a IP over IP packet which has
inner IPv6 header. Thus the correct Tx context descriptor cannot be
composed, resulting in Tx ring hang.

Fixes: 3403960cdf86 ("net: wangxun: libwx add tx offload functions")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Link: https://patch.msgid.link/20250324103235.823096-1-jiawenwu@trustnetic.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>