netfilter: ctnetlink: export nf_conntrack_max

IPCTNL_MSG_CT_GET_STATS netlink command allow to monitor current number
of conntrack entries. However, if one wants to compare it with the
maximum (and detect exhaustion), the only solution is currently to read
sysctl value.

This patch add nf_conntrack_max value in netlink message, and simplify
monitoring for application built on netlink API.

Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>