netgraph/ng_base: Renaming a node to the same name is a noop

Detailed analysis in https://github.com/genneko/freebsd-vimage-jails/issues/2
brought the problem down to a double call of ng_node_name() before and
after a vnet move. Because the name of the node is already known
(occupied by itself), the second call fails.

PR: 241954
Reported by: Paul Armstrong
MFC: 1 week
Differential Revision: https://reviews.freebsd.org/D30110