cipso: correct comments of cipso_v4_cache_invalidate()

Since cipso_v4_cache_invalidate() has no return value, so drop
related descriptions in its comments.

Fixes: 446fda4f ("[NetLabel]: CIPSOv4 engine")
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: David S. Miller <davem@davemloft.net>