genetlink: constify genl_err_attr() argument

genl_err_attr() sets netlink_ext_ack::bad_attr which is a pointer to const
struct nlattr so make the attr argument also const.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>