Description: tun: correct header offsets in napi frags mode
References:
 https://source.android.com/security/bulletin/pixel/2021-01-01
Notes:
 carnil> Commit fixes 90e33d459407 ("tun: enable napi_gro_frags() for
 carnil> TUN/TAP driver") in 4.15-rc1.
Bugs:
upstream: released (5.8-rc1) [96aa1b22bd6bb9fccf62f6261f390ed6f3e7967f]
5.10-upstream-stable: N/A "Fixed before branching point"
4.19-upstream-stable: released (4.19.129) [75e36c19ff10836e5d03f87cf17793cf83b59430]
4.9-upstream-stable: N/A "Vulnerable code not present"
sid: released (5.7.6-1)
4.19-buster-security: released (4.19.131-1)
4.9-stretch-security: N/A "Vulnerable code not present"