ip6_tunnel: Fix broken GRO

[ Upstream commit 4b3e33fcc38f7750604b065c55a43e94c5bc3145 ]

GRO code checks for matching layer 2 headers to see, if packet belongs
to the same flow and because ip6 tunnel set dev->hard_header_len
this check fails in cases, where it shouldn't. To fix this don't
set hard_header_len, but use needed_headroom like ipv4/ip_tunnel.c
does.

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Link: https://patch.msgid.link/20240815151419.109864-1-tbogendoerfer@suse.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>