Merge branch 'selftests-net-more-pmtu-sh-fixes'

Paolo Abeni says:

====================
selftests: net: more pmtu.sh fixes

The mentioned test is still flaky, unusally enough in 'fast'
environments.

Patch 2/2 [try to] address the existing issues, while patch 1/2
introduces more strict tests for the existing net helpers, to hopefully
prevent future pain.
====================

Link: https://lore.kernel.org/r/cover.1707731086.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>