kqueue: fix incorrect mutex used in Close()
kqueue: fix incorrect mutex used in Close()

identified at https://code.google.com/p/go/issues/detail?id=8282#c5

mingshun/fsnotify,treejames/fsnotify,fsnotify/fsnotify,scalingdata/fsnotify-v1,omeid/fsnotify,beni55/fsnotify,fsnotify/fsnotify,jsix/fsnotify,fsnotify/fsnotify,qida/fsnotify