kernel: scale nf_conntrack_max more reasonably

Use the kernel's built-in formula for computing this value.
The value applied by OpenWRT's sysctl configuration file does not scale
with the available memory, under-using hardware capabilities.
Also, that formula also influences net.netfilter.nf_conntrack_buckets,
which should improve conntrack performance in average (fewer connections
per hashtable bucket).

Backport upstream commit for its effect on the number of connections per
hashtable bucket.

Apply a hack patch to set the RAM size divisor to a more reasonable value (2048,
down from 16384) for our use case, a typical router handling several thousands
of connections.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
(cherry picked from commit 15fbb91666)

target/linux/generic/hack-5.10/661-kernel-ct-size-the-hashtable-more-adequately.patch

@@ -0,0 +1,25 @@
+From 804fbb3f2ec9283f7b778e057a68bfff440a0be6 Mon Sep 17 00:00:00 2001
+From: Rui Salvaterra <rsalvaterra@gmail.com>
+Date: Wed, 30 Mar 2022 22:51:55 +0100
+Subject: [PATCH] kernel: ct: size the hashtable more adequately
+
+To set the default size of the connection tracking hash table, a divider of
+16384 becomes inadequate for a router handling lots of connections. Divide by
+2048 instead, making the default size scale better with the available RAM.
+
+Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
+---
+ net/netfilter/nf_conntrack_core.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/net/netfilter/nf_conntrack_core.c
++++ b/net/netfilter/nf_conntrack_core.c
+@@ -2576,7 +2576,7 @@ int nf_conntrack_init_start(void)
+ 
+ 	if (!nf_conntrack_htable_size) {
+ 		nf_conntrack_htable_size
+-			= (((nr_pages << PAGE_SHIFT) / 16384)
++			= (((nr_pages << PAGE_SHIFT) / 2048)
+ 			   / sizeof(struct hlist_head));
+ 		if (BITS_PER_LONG >= 64 &&
+ 		    nr_pages > (4 * (1024 * 1024 * 1024 / PAGE_SIZE)))