Continue with the loop after writing to ctrlStream

[tedyu]

#11754 fixed a bug where watch stream was prematurely closed.

However, when the write to sws.ctrlStream succeeds, we should continue with the for loop. The existing code doesn't achieve this purpose.

This PR makes the continue effective by introducing label.

/cc @polyrabbit @xiang90 @jingyih

[polyrabbit]

I suppose the continue itself is enough to continue with the for loop

etcd/etcdserver/api/v3rpc/watch.go

Lines 257 to 260 in 73b936b

	case sws.ctrlStream <- wr:
	continue
	case <-sws.closec:
	return nil