Description: l2tp: pass correct message length to ip6_append_data
References:
Notes:
 carnil> Introduced in 9d4c75800f61 ("ipv4, ipv6: Fix handling of transhdrlen in
 carnil> __ip{,6}_append_data()"). Vulnerable versions: 4.14.327 4.19.296 5.4.258
 carnil> 5.10.198 5.15.135 6.1.57 6.5.7 6.6-rc5.
Bugs:
upstream: released (6.8-rc6) [359e54a93ab43d32ee1bff3c2f9f10cb9f6b6e79]
6.7-upstream-stable: released (6.7.7) [83340c66b498e49353530e41542500fc8a4782d6]
6.6-upstream-stable: released (6.6.19) [804bd8650a3a2bf3432375f8c97d5049d845ce56]
6.1-upstream-stable: released (6.1.80) [13cd1daeea848614e585b2c6ecc11ca9c8ab2500]
5.10-upstream-stable: released (5.10.211) [dcb4d14268595065c85dc5528056713928e17243]
4.19-upstream-stable: released (4.19.308) [4c3ce64bc9d36ca9164dd6c77ff144c121011aae]
sid: released (6.7.7-1)
6.1-bookworm-security: released (6.1.82-1)
5.10-bullseye-security: needed
4.19-buster-security: needed