Merge branch 'selftests-pmtu-sh-improve-the-test-result-processing'

Po-Hsu Lin says:

====================
selftests: pmtu.sh: improve the test result processing

The pmtu.sh test script treats all non-zero return code as a failure,
thus it will be marked as FAILED when some sub-test got skipped.

This patchset will:
  1. Use the kselftest framework skip code $ksft_skip to replace the
     hardcoded SKIP return code.
  2. Improve the result processing, the test will be marked as PASSED
     if nothing goes wrong and not all the tests were skipped.
====================

Link: https://lore.kernel.org/r/20201110020049.6705-1-po-hsu.lin@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>