tracing: Have trace_array keep track if snapshot buffer is allocated

The snapshot buffer belongs to the trace array not the tracer that is
running. The trace array should be the data structure that keeps track
of whether or not the snapshot buffer is allocated, not the tracer
desciptor. Having the trace array keep track of it makes modifications
so much easier.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>