kernel revocation list check during boot fails on Trusty (old keyctl?)

Boot test of 2021.11.29/trusty/linux-azure/4.15.0-1128.141~14.04.1 failed on kernel_revocation_list check with:

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19:21:35 INFO | START ubuntu_boot.kernel_revocation_list ubuntu_boot.kernel_revocation_list timestamp=1638559295 timeout=300 localtime=Dec 03 19:21:35
19:21:35 DEBUG| Persistent state client._record_indent now set to 2
19:21:35 DEBUG| Persistent state client.unexpected_reboot now set to ('ubuntu_boot.kernel_revocation_list', 'ubuntu_boot.kernel_revocation_list')
19:21:35 DEBUG| Waiting for pid 2224 for 300 seconds
19:21:36 INFO | Checking kernel revocation list
19:21:36 DEBUG| Running 'python3 /home/azure/autotest/client/tests/ubuntu_boot/kernel_revocation_list.py'
19:21:36 DEBUG| [stderr] test_revocations (__main__.TestRevocationList) ... Unparsable key: '%:.blacklist'
19:21:36 DEBUG| [stderr] ERROR
19:21:36 DEBUG| [stderr]
19:21:36 DEBUG| [stderr] ======================================================================
19:21:36 DEBUG| [stderr] ERROR: test_revocations (__main__.TestRevocationList)
19:21:36 DEBUG| [stderr] ----------------------------------------------------------------------
19:21:36 DEBUG| [stderr] Traceback (most recent call last):
19:21:36 DEBUG| [stderr] File "/home/azure/autotest/client/tests/ubuntu_boot/kernel_revocation_list.py", line 27, in test_revocations
19:21:36 DEBUG| [stderr] ["keyctl", "list", "%:.blacklist"], universal_newlines=True
19:21:36 DEBUG| [stderr] File "/usr/lib/python3.4/subprocess.py", line 620, in check_output
19:21:36 DEBUG| [stderr] raise CalledProcessError(retcode, process.args, output=output)
19:21:36 DEBUG| [stderr] subprocess.CalledProcessError: Command '['keyctl', 'list', '%:.blacklist']' returned non-zero exit status 2
19:21:36 DEBUG| [stderr]
19:21:36 DEBUG| [stderr] ----------------------------------------------------------------------
19:21:36 DEBUG| [stderr] Ran 1 test in 0.013s
19:21:36 DEBUG| [stderr]
19:21:36 DEBUG| [stderr] FAILED (errors=1)
19:21:36 ERROR| Exception escaping from test:
Traceback (most recent call last):
File "/home/azure/autotest/client/shared/test.py", line 411, in _exec
_call_test_function(self.execute, *p_args, **p_dargs)
File "/home/azure/autotest/client/shared/test.py", line 823, in _call_test_function
return func(*args, **dargs)
File "/home/azure/autotest/client/shared/test.py", line 291, in execute
postprocess_profiled_run, args, dargs)
File "/home/azure/autotest/client/shared/test.py", line 212, in _call_run_once
self.run_once(*args, **dargs)
File "/home/azure/autotest/client/tests/ubuntu_boot/ubuntu_boot.py", line 79, in run_once
raise error.TestFail()
TestFail
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