ndp tests: fix cleanup

Fix the cleanup function name so we actually remove our test jail and
interfaces.

Sponsored by:	Rubicon Communications, LLC ("Netgate")

tests/sys/netinet6/ndp.sh

@@ -99,7 +99,7 @@ ndp_del_gu_success_body() {
 	atf_check -o match:"2001:db8::2 \(2001:db8::2\) deleted" jexec ${jname} ndp -nd 2001:db8::2
 }
 
-ndp_del_success_cleanup() {
+ndp_del_gu_success_cleanup() {
 	vnet_cleanup
 }