netfilter: conntrack: Use max() instead of doing it manually

Fix following coccicheck warning:

./include/net/netfilter/nf_conntrack.h:282:16-17: WARNING opportunity
for max().

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>