netfilter: conncount: Support count only use case

Currently, nf_conncount_count() counts the number of connections that
matches key and inserts a conntrack 'tuple' with the same key into the
accounting data structure.  This patch supports another use case that only
counts the number of connections where 'tuple' is not provided.  Therefore,
proper changes are made on nf_conncount_count() to support the case where
'tuple' is NULL.  This could be useful for querying statistics or
debugging purpose.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>