odp-util: Only add recirc_id mask to Netlink message if mask is provided

Current unconditional call may result in NULL being passed to
nl_msg_put_u32().

Cc: Andy Zhou <azhou@nicira.com>
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>