netfilter: conntrack: disable 0 value for conntrack_max setting

Undocumented historical artifact inherited from ip_conntrack.
If value is 0, then no limit is applied at all, conntrack table
can grow to huge value, only limited by size of conntrack hashes and
the kernel-internal upper limit on the hash chain lengths.

This feature makes no sense; users can just set
conntrack_max=2147483647 (INT_MAX).

Disallow a 0 value. This will make it slightly easier to allow
per-netns constraints for this value in a future patch.

Signed-off-by: Florian Westphal <[e-mail]>

net/netfilter/nf_conntrack_core.c

···
 	/* We don't want any race condition at early drop stage */
 	ct_count = atomic_inc_return(&cnet->count);
 
-	if (nf_conntrack_max && unlikely(ct_count > nf_conntrack_max)) {
+	if (unlikely(ct_count > nf_conntrack_max)) {
 		if (!early_drop(net, hash)) {
 			if (!conntrack_gc_work.early_drop)
 				conntrack_gc_work.early_drop = true;

net/netfilter/nf_conntrack_standalone.c

···
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= SYSCTL_ZERO,
+		.extra1		= SYSCTL_ONE,
 		.extra2		= SYSCTL_INT_MAX,
 	},
 	[NF_SYSCTL_CT_COUNT] = {
···
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= SYSCTL_ZERO,
+		.extra1		= SYSCTL_ONE,
 		.extra2		= SYSCTL_INT_MAX,
 	},
 };