tracing: Add missing nest end to synth_event_trace_start() error case

If the ring_buffer reserve in synth_event_trace_start() fails, the
matching ring_buffer_nest_end() should be called in the error code,
since nothing else will ever call it in this case.

Link: http://lkml.kernel.org/r/20abc444b3eeff76425f895815380abe7aa53ff8.1581374549.git.zanussi@kernel.org

Fixes: 8dcc53ad ("tracing: Add synth_event_trace() and related functions")
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -2043,6 +2043,7 @@ int synth_event_trace_start(struct trace_event_file *file,
 	entry = trace_event_buffer_reserve(&trace_state->fbuffer, file,
 					   sizeof(*entry) + fields_size);
 	if (!entry) {
+		ring_buffer_nest_end(trace_state->buffer);
 		ret = -EINVAL;
 		goto out;
 	}