Fix: baddr_statedump tracepoint registration

Make sure that the ust_baddr_statedump probe is registered prior to
using the tracepoint in lttng_ust_baddr_statedump(). This fix solves the
issue that in rare cases the very first ust_baddr_statedump events were
missing.

Use a reference counting approach that allows constructors/destructors
of the probe to be called many times, as long as the number of calls to
constructor matches the number of calls to destructor.

Reported-by: Paul Woegerer <paul_woegerer@mentor.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>