Merge branch 'selftests-net-a-few-pmtu-sh-fixes'

Paolo Abeni says:

====================
selftests: net: a few pmtu.sh fixes

This series try to address CI failures for the pmtu.sh tests. It
does _not_ attempt to enable all the currently skipped cases, to
avoid adding more entropy.

Tested with:

make -C tools/testing/selftests/ TARGETS=net install
vng --build  --config tools/testing/selftests/net/config
vng --run . --user root -- \
	./tools/testing/selftests/kselftest_install/run_kselftest.sh \
	-t net:pmtu.sh
====================

Link: https://lore.kernel.org/r/cover.1706635101.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>