netfilter: conntrack: avoid same-timeout update

No need to dirty a cache line if timeout is unchanged.
Also, WARN() is useless here: we crash on 'skb->len' access
if skb is NULL.

Last, ct->timeout is u32, not 'unsigned long' so adapt the
function prototype accordingly.

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