Merge branch 'tunnels-fix-ipv4-pmtu-icmp-checksum'

Florian Westphal says:

====================
tunnels: fix ipv4 pmtu icmp checksum

The checksum of the generated ipv4 icmp pmtud message is
only correct if the skb that causes the icmp error generation
is linear.

Fix this and add a selftest for this.
====================

Link: https://lore.kernel.org/r/20230803152653.29535-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>