netfilter: cttimeout: inc/dec module refcount per object, not per use refcount

There is no need to increment the module refcount again, its enough to
obtain one reference per object, i.e. take a reference on object
creation and put it on object destruction.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>