[][openwrt][mt7988][crypto][Refactor kernel patch to decouple IPSec and hnat]

[Description]
Refactor kernel patch to decouple IPSec and hnat.
Add the structure for skb headroom in mtk_eth_soc.h when hnat module is
not build. So, IPSec can still use headroom to send packets to EIP197.

[Release-log]
N/A


Change-Id: Ifb6a53c343a4b0d5114e44eb6dbb3edbed97b3ad
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8872659
1 file changed