When a user passes a negative value for clone_skb, the num_args() function stops parsing at the first nonnumeric value. This makes it impossible for clone_skb to be set to a negative value. For example: "clone_skb -200" would stop parsing at the first char ('-') and return zero for the new clone_skb value.
So to answer your question: No clone_skb cannot be negative.
Apologies for the email format errors and thanks for the quick reply!