pkg/filenotify/poller: fix Close()

The code in Close() that removes the watches was not working,
because it first sets `w.closed = true` and then calls w.close(),
which starts with
```
        if w.closed {
                return errPollerClosed
	}
```

Fix by setting w.closed only after calling w.remove() for all the
files being watched.

While at it, remove the duplicated `delete(w.watches, name)` code.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/filenotify/poller.go

@@ -115,11 +115,10 @@ func (w *filePoller) Close() error {
 		return nil
 	}
 
-	w.closed = true
 	for name := range w.watches {
 		w.remove(name)
-		delete(w.watches, name)
 	}
+	w.closed = true
 	return nil
 }