Backport initialization of mu in NewCtxClient to release-3.5

Tested against the demo code provided in PR etcd-io#17001

References:
- etcd-io#17001
- etcd-io#17018

Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>

client/v3/client.go

@@ -86,7 +86,7 @@ func New(cfg Config) (*Client, error) {
 // service interface implementations and do not need connection management.
 func NewCtxClient(ctx context.Context, opts ...Option) *Client {
 	cctx, cancel := context.WithCancel(ctx)
-	c := &Client{ctx: cctx, cancel: cancel, lgMu: new(sync.RWMutex)}
+	c := &Client{ctx: cctx, cancel: cancel, lgMu: new(sync.RWMutex), mu: new(sync.RWMutex)}
 	for _, opt := range opts {
 		opt(c)
 	}