[#3294] fix reservation-del psql query

- fixing a bug where when postgresql used as hosts reservations backend and
  when trying to delete only one reservation with host_cmds reservation-del
  by ipv6 and subnet-id, all host reservations in given subnet were deleted

Author: pzadroga

src/lib/dhcpsrv/pgsql_host_data_source.cc

@@ -2191,7 +2191,8 @@ TaggedStatementArray tagged_statements = { {
      { OID_INT8, OID_VARCHAR },
      "del_host_addr6",
      "DELETE FROM hosts USING ipv6_reservations "
-     "  WHERE dhcp6_subnet_id = $1 AND ipv6_reservations.address = cast($2 as inet)"
+     "  WHERE hosts.host_id = ipv6_reservations.host_id"
+     "  AND dhcp6_subnet_id = $1 AND ipv6_reservations.address = cast($2 as inet)"
     },
 
     // PgSqlHostDataSourceImpl::DEL_HOST_SUBID4_ID