vlib: autogenerate <node> before <last-in-arc> constraints

If an arc declaration includes '.last_in_arc = "some-node"', assume
that folks mean it and add explicit ordering constraints.

Fix the "arp" arc declaration which claimed that the arc ends at
arp-disabled, but the arc really ends at error-drop.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Ie2de1fb30091671cbc7c62770903a2e05987f141
diff --git a/src/vnet/ethernet/arp.c b/src/vnet/ethernet/arp.c
index a42cfda..ca9a21c 100644
--- a/src/vnet/ethernet/arp.c
+++ b/src/vnet/ethernet/arp.c
@@ -1711,7 +1711,7 @@
 {
   .arc_name = "arp",
   .start_nodes = VNET_FEATURES ("arp-input"),
-  .last_in_arc = "arp-disabled",
+  .last_in_arc = "error-drop",
   .arc_index_ptr = &ethernet_arp_main.feature_arc_index,
 };