cls_cgroup: Initialise classid when module is absent

When the cls_cgroup module is not loaded, task_cls_classid will
return an uninitialised classid instead of zero.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>