Comment 15 for bug 1772775

Its possible the following commit is the fix:
f4eb17e1efe5 ("Revert "sit: reload iphdr in ipip6_rcv"")

This commit was added to mainline in v4.12-rc5, which could be why this bug was not seen in Artful or newer.

I built a test kernel with commit f4eb17e1efe5. The test kernel can be downloaded from:
http://kernel.ubuntu.com/~jsalisbury/lp1772775

Can you test this kernel and see if it resolves this bug?