selftests: pmtu.sh: improve the test result processing

This test will treat all non-zero return codes as failures, it will
make the pmtu.sh test script being marked as FAILED when some
sub-test got skipped.

Improve the result processing by
  * Only mark the whole test script as SKIP when all of the
    sub-tests were skipped
  * If the sub-tests were either passed or skipped, the overall
    result will be PASS
  * If any of them has failed with return code 1 or anything bad
    happened (e.g. return code 127 for command not found), the
    overall result will be FAIL

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/pmtu.sh

@@ -1652,7 +1652,19 @@ run_test() {
 	return $ret
 	)
 	ret=$?
-	[ $ret -ne 0 ] && exitcode=1
+	case $ret in
+		0)
+			all_skipped=false
+			[ $exitcode=$ksft_skip ] && exitcode=0
+		;;
+		$ksft_skip)
+			[ $all_skipped = true ] && exitcode=$ksft_skip
+		;;
+		*)
+			all_skipped=false
+			exitcode=1
+		;;
+	esac
 
 	return $ret
 }
@@ -1786,6 +1798,7 @@ usage() {
 #
 exitcode=0
 desc=0
+all_skipped=true
 
 while getopts :ptv o
 do