tracing: Annotate implicit fall through in parse_probe_arg()

There is a plan to build the kernel with -Wimplicit-fallthrough and
this place in the code produced a warning (W=1).

This commit remove the following warning:

  kernel/trace/trace_probe.c:302:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Link: http://lkml.kernel.org/r/20190114203039.16535-1-malat@debian.orgSigned-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_probe.c

@@ -300,6 +300,7 @@ parse_probe_arg(char *arg, const struct fetch_type *type,
 
 	case '+':	/* deref memory */
 		arg++;	/* Skip '+', because kstrtol() rejects it. */
+		/* fall through */
 	case '-':
 		tmp = strchr(arg, '(');
 		if (!tmp)