fsnotify: Do not return merged event from fsnotify_add_notify_event()

fsnotify: Do not return merged event from fsnotify_add_notify_event()

The event returned from fsnotify_add_notify_event() cannot ever be used
safely as the event may be freed by the time the function returns (after
dropping notification_mutex). So change the prototype to just return
whether the event was added or merged into some existing event.

Reported-and-tested-by: Jiri Kosina <[email protected]>
Reported-and-tested-by: Dave Jones <[email protected]>
Signed-off-by: Jan Kara <[email protected]>