ubuntu_ftrace_smoke_test hangs on Azure on v5.4

[Krzysztof Kozlowski]

ubuntu_ftrace_smoke_test on all v5.4 kernels on all Azure instances hangs:

----
114 11:22:52 INFO | Writing results to /home/azure/autotest/client/results/default
115 11:22:52 DEBUG| Initializing the state engine
116 11:22:52 DEBUG| Persistent state client.steps now set to []
117 11:22:52 DEBUG| Persistent option harness now set to None
118 11:22:52 DEBUG| Persistent option harness_args now set to None
119 11:22:52 DEBUG| Selected harness: standalone
120 11:22:52 INFO | START ---- ---- timestamp=1631791372 localtime=Sep 16 11:22:52
121 11:22:52 DEBUG| Persistent state client._record_indent now set to 1
122 11:22:52 DEBUG| Test has timeout: 900 sec.
123 11:22:52 INFO | START ubuntu_ftrace_smoke_test.ftrace-smoke-test ubuntu_ftrace_smoke_test.ftrace-smoke-test timestamp=1631791372 timeout=900 localtime=Sep 16 11:22:52
124 11:22:52 DEBUG| Persistent state client._record_indent now set to 2
125 11:22:52 DEBUG| Persistent state client.unexpected_reboot now set to ('ubuntu_ftrace_smoke_test.ftrace-smoke-test', 'ubuntu_ftrace_smoke_test.ftrace-smoke-test')
126 11:22:52 DEBUG| Waiting for pid 2953 for 900 seconds
127 11:22:52 DEBUG| Running '/home/azure/autotest/client/tests/ubuntu_ftrace_smoke_test/ubuntu_ftrace_smoke_test.sh'
128 11:22:52 DEBUG| [stdout] PASSED (CONFIG_FUNCTION_TRACER=y in /boot/config-5.4.0-1059-azure)
129 11:22:52 DEBUG| [stdout] PASSED (CONFIG_FUNCTION_GRAPH_TRACER=y in /boot/config-5.4.0-1059-azure)
130 11:22:52 DEBUG| [stdout] PASSED (CONFIG_STACK_TRACER=y in /boot/config-5.4.0-1059-azure)
131 11:22:52 DEBUG| [stdout] PASSED (CONFIG_DYNAMIC_FTRACE=y in /boot/config-5.4.0-1059-azure)
132 11:22:54 DEBUG| [stdout] PASSED all expected /sys/kernel/debug/tracing files exist
133 11:22:54 DEBUG| [stdout] PASSED (function_graph in /sys/kernel/debug/tracing/available_tracers)
134 11:22:54 DEBUG| [stdout] PASSED (function in /sys/kernel/debug/tracing/available_tracers)
135 11:22:54 DEBUG| [stdout] PASSED (nop in /sys/kernel/debug/tracing/available_tracers)
136 11:22:57 DEBUG| [stdout] PASSED (tracer function can be enabled)
----

Found on Azure clouds:
* focal/linux/5.4.0-85.95
* focal/linux-azure/5.4.0-1059.62
* bionic/linux-azure-5.4/5.4.0-1059.62~18.04.1
* SRU cycles: 2021.08.16, 2021.07.19, 2021.05.31 (not checked earlier)

Not found:
* Other clouds.
* Azure: hirsute/linux/5.11.0-35.37
* Azure: focal/linux-azure-5.11/5.11.0-1016.17~20.04.1
* Azure: focal/linux-azure-5.8/5.8.0-1042.45~20.04.1
* Azure: bionic/linux-azure-4.15/4.15.0-1124.137