Merge branch 'more-pmtu-selftests'

Sabrina Dubroca says:

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selftests: add more PMTU tests

The current selftests for PMTU cover VTI tunnels, but there's nothing
about the generation and handling of PMTU exceptions by intermediate
routers. This series adds and improves existing helpers, then adds
IPv4 and IPv6 selftests with a setup involving an intermediate router.

Joint work with Stefano Brivio.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>