tracing: Have COMM event filter key be treated as a string

The GLOB operation "~" should be able to work with the COMM filter key in
order to trace programs with a glob. For example

  echo 'COMM ~ "systemd*"' > events/syscalls/filter
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -1312,7 +1312,8 @@ static inline bool is_string_field(struct ftrace_event_field *field)
 {
 	return field->filter_type == FILTER_DYN_STRING ||
 	       field->filter_type == FILTER_STATIC_STRING ||
-	       field->filter_type == FILTER_PTR_STRING;
+	       field->filter_type == FILTER_PTR_STRING ||
+	       field->filter_type == FILTER_COMM;
 }
 
 static inline bool is_function_field(struct ftrace_event_field *field)