fbt(4): allow tracing in dtrace_test.ko if it was loaded after fbt

[christos]

Summary

This is a workaround (needed by a test script in D38825) for an as-of-yet
unknown bug. Basically, FBT does not allow tracing of dtrace_test.ko
functions if dtrace_test.ko was loaded after fbt.ko, while it can trace
them perfectly fine if it's loaded before fbt.ko.