Ensure ipv4 nat entry is active

ensure_ipv4_nat_entry previously would match against *any* existing
table entries with the matching parameters. We need it to only
match against entries that are *active*, or in implementation terms,
entries whose version_removed column is NULL.

nexus/db-queries/src/db/datastore/ipv4_nat_entry.rs

@@ -36,6 +36,7 @@ impl DataStore {
             .filter(dsl::sled_address.eq(nat_entry.sled_address))
             .filter(dsl::vni.eq(nat_entry.vni))
             .filter(dsl::mac.eq(nat_entry.mac))
+            .filter(dsl::version_removed.is_null())
             .select((
                 dsl::external_address,
                 dsl::first_port,