IPv6/GRO: generic helper to remove temporary HBH/jumbo header in driver

IPv6/TCP and GRO stacks can build big TCP packets with an added
temporary Hop By Hop header.

Is GSO is not involved, then the temporary header needs to be removed in
the driver. This patch provides a generic helper for drivers that need
to modify their headers in place.

Tested:
Compiled and ran with ethtool -K eth1 tso off
Could send Big TCP packets

Signed-off-by: Coco Li <lixiaoyan@google.com>
Link: https://lore.kernel.org/r/20221210041646.3587757-1-lixiaoyan@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>