When anything in systemtap_module_init fails, and we return non-zero,
then the module load is aborted. The normal module unload path
(systemtap_module_exit) is not even attempted, so we need to make sure
that all partially-allocated resources are returned.
Our timer callbacks for the gettimeofday subsystem are a classic example
of this error. If we don't unregister the timers before aborting init,
they will later be called and cause a kernel fault.
We also were neglecting to free the percpu context. A memory leak is
less harmful, but that's fixed now too.