stable request: Revert "sit: reload iphdr in ipip6_rcv"

Hello,

Please consider backporting to 4.9.y the following commit from DaveM [CC'ed]:

f4eb17e1efe538d4da7d574bedb00a8dafcc26b7 ("Revert "sit: reload iphdr in ipip6_rcv"")

It cherry-picks cleanly and builds fine.

The original commit was introduced and reverted in v4.12-rc5, but only the original made its way into the 4.9 stable release (v4.9.94).

It causes a regression: pings to v6 hosts over a SIT tunnel do not work anymore, and the following error appears in dmesg:

kernel: sit: non-ECT from 0.0.0.0 with TOS=0xd

This was also reported and reverted downstream by Ubuntu:

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1772775

Thanks!