pmtu.sh net selftest fail with PMTU exception wasn't created after exceeding MTU

[John Cabaj]

Issue found on J-gcp-6.5.0-1015.15~22.04.1 and M-gcp-6.5.0-1015.15 in cycle sru-20240205

And can be reproduced with generic kernel on GCP cloud.

This is not a regression but something emerges after the pmtu.sh result parsing logic update, which landed via stable update in sru-20240205 cycle (bug 2043198)

Dive into test history, we can see this failure all the way back to sru-20231030 with M-6.5.0-1010.10 on GCP cloud.

19:48:21 DEBUG| [stdout] # selftests: net: pmtu.sh
19:48:21 DEBUG| [stdout] # TEST: ipv4: PMTU exceptions [ OK ]
19:48:21 DEBUG| [stdout] # TEST: ipv4: PMTU exceptions - nexthop objects [ OK ]
19:48:22 DEBUG| [stdout] # TEST: ipv6: PMTU exceptions [ OK ]
19:48:22 DEBUG| [stdout] # TEST: ipv6: PMTU exceptions - nexthop objects [ OK ]
19:48:23 DEBUG| [stdout] # TEST: ICMPv4 with DSCP and ECN: PMTU exceptions [FAIL]
19:48:23 DEBUG| [stdout] # PMTU exception wasn't created after exceeding MTU
19:48:24 DEBUG| [stdout] # TEST: ICMPv4 with DSCP and ECN: PMTU exceptions - nexthop objects [FAIL]
19:48:24 DEBUG| [stdout] # PMTU exception wasn't created after exceeding MTU
19:48:25 DEBUG| [stdout] # TEST: UDPv4 with DSCP and ECN: PMTU exceptions [FAIL]
19:48:25 DEBUG| [stdout] # PMTU exception wasn't created after exceeding MTU
19:48:25 DEBUG| [stdout] # TEST: UDPv4 with DSCP and ECN: PMTU exceptions - nexthop objects [FAIL]
19:48:25 DEBUG| [stdout] # PMTU exception wasn't created after exceeding MTU
....

Logfile also attached (rt.log)