tracing: Add generic event-name based remove event method

Add a generic method to remove event from dynamic event
list. This is same as other system under ftrace. You
just need to pass the event name with '!', e.g.

# echo p:new_grp/new_event _do_fork > dynamic_events

This creates an event, and

# echo '!p:new_grp/new_event _do_fork' > dynamic_events

Or,

# echo '!p:new_grp/new_event' > dynamic_events

will remove new_grp/new_event event.

Note that this doesn't check the event prefix (e.g. "p:")
strictly, because the "group/event" name must be unique.

Link: http://lkml.kernel.org/r/154140869774.17322.8887303560398645347.stgit@devbox

Reviewed-by: Tom Zanussi <[e-mail]>
Tested-by: Tom Zanussi <[e-mail]>
Signed-off-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

kernel/trace/trace_dynevent.c

···
 	char *system = NULL, *event, *p;
 	int ret = -ENOENT;
 
-	if (argv[0][1] != ':')
-		return -EINVAL;
+	if (argv[0][0] == '-') {
+		if (argv[0][1] != ':')
+			return -EINVAL;
+		event = &argv[0][2];
+	} else {
+		event = strchr(argv[0], ':');
+		if (!event)
+			return -EINVAL;
+		event++;
+	}
 
-	event = &argv[0][2];
 	p = strchr(event, '/');
 	if (p) {
 		system = event;
···
 	struct dyn_event_operations *ops;
 	int ret;
 
-	if (argv[0][0] == '-')
+	if (argv[0][0] == '-' || argv[0][0] == '!')
 		return dyn_event_release(argc, argv, NULL);
 
 	mutex_lock(&dyn_event_ops_mutex);