clientconn: use separate defer blocks

Commit 955eb8a ("channelz: cleanup channel registration if Dial
fails (grpc#2733)") moved a defer block earlier in DialContext() to
ensure that cc.Close() was always called.  This defer block also
checks whether the ctx.Done() is true, and if so ensures the context
error is returned.

If the dial options include a timeout, the original context gets
replaced with a new context that has the timeout, and this gets
a catchall `defer cancel()` to go with it.

However, this cancel() now gets called before the cleanup defer
block, so when the latter runs the context is always already cancelled.

Fix by splitting the larger defer block into two parts:
 - The part that does cc.Close() stays near the beginning of the
   method.
 - The part that checks ctx.Done() returns to below the `defer cancel()`
   call, and so gets invoked before it.

clientconn.go

@@ -135,12 +135,6 @@ func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *
 	}
 
 	defer func() {
-		select {
-		case <-ctx.Done():
-			conn, err = nil, ctx.Err()
-		default:
-		}
-
 		if err != nil {
 			cc.Close()
 		}
@@ -207,6 +201,13 @@ func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *
 		ctx, cancel = context.WithTimeout(ctx, cc.dopts.timeout)
 		defer cancel()
 	}
+	defer func() {
+		select {
+		case <-ctx.Done():
+			conn, err = nil, ctx.Err()
+		default:
+		}
+	}()
 
 	scSet := false
 	if cc.dopts.scChan != nil {