It looks like struct stacktrace_ops is no longer there in rhel7.5 so our detection of if dump_stack would work is no longer working.
We get this now:
[29185.979935] LNet: Service thread pid 9373 was inactive for 40.04s. The thread might be hung, or it might only be slow and will resume later. Dumping the stack trace for debugging purposes:
[29185.981691] LNet: 31237:0:(linux-debug.c:185:libcfs_call_trace()) can't show stack: kernel doesn't export show_task
[29185.982826] LustreError: dumping log to /tmp/lustre-log.1527605717.9373
Given that struct stacktrace_ops are still there in mainline kernels, this seems something specific to rhel7.5 and we need to find another way of detecting this I guess.