$ cc -O2 -o tty_leak2 tty_leak2.c
(no output; exit 0)
$ cd /usr/src/sys/netgraph7/tty && make   # with fix.diff applied
cc ... -c ng_tty.c
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o ng_tty.ko ng_tty.o
TTY_BUILT
$ grep -n NG_NODE_UNREF /usr/src/sys/netgraph7/tty/ng_tty.c
214:		NG_NODE_UNREF(sc->node);          # pre-existing open-error path
570:	NG_NODE_UNREF(node);	/* drop the constructor reference (DF-3534) */