lib/nfs/Connection: fix memory leak (and assertion failure)

nfs_destroy_context() will invoke all pending callbacks with
err==-EINTR.  In CancellableCallback::Callback(), this will invoke
NfsConnection::DeferClose(), which however is only designed to be
called from nfs_service().  In non-debug mode, this will leak memory
because nfs_close_async() is never called.

Workaround: before nfs_destroy_context(), invoke nfs_close_async() on
all pending file handles.

src/lib/nfs/Cancellable.hxx

@@ -157,6 +157,12 @@ public:
 
 		return *i;
 	}
+
+	template<typename F>
+	void ForEach(F &&f) {
+		for (CT &i : list)
+			f(i);
+	}
 };
 
 #endif