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.
(cherry picked from commit 0345fd891fe13a191fc0fae9463ea9458bfaff5a)