ndb: make non-unique FDB index more unique

There can be multicast FDBs with single ifindex and no NDA_VLAN
at all (that means NDA_VLAN = 0), but NDA_DST will be different.

pyroute2/ndb/objects/neighbour.py

@@ -48,8 +48,9 @@ def load_ndmsg(schema, target, event):
 
 brmsg_schema = (
     ndmsg.sql_schema()
-    .unique_index('ifindex', 'NDA_LLADDR', 'NDA_VLAN')
+    .unique_index('ifindex', 'NDA_LLADDR', 'NDA_DST', 'NDA_VLAN')
     .constraint('NDA_LLADDR', "NOT NULL DEFAULT ''")
+    .constraint('NDA_DST', "NOT NULL DEFAULT ''")
     .constraint('NDA_VLAN', "NOT NULL DEFAULT 0")
     .foreign_key(
         'interfaces',