Reintroduce IPv6 reachability probe patch for RPi lost after refactoring

buildroot-external/patches/linux/0001-ipv6-add-option-to-explicitly-enable-reachability-te.patch

@@ -0,0 +1,52 @@
+From 76591e4075194cf717dc085b8285912f706bcd46 Mon Sep 17 00:00:00 2001
+From: Stefan Agner <[email protected]>
+Date: Tue, 28 Mar 2023 12:02:10 +0200
+Subject: [PATCH] ipv6: add option to explicitly enable reachability test
+
+Systems which act as host as well as router might prefer the host
+behavior. Currently the kernel does not allow to use IPv6 forwarding
+globally and at the same time use route reachability probing.
+
+Add a compile time flag to enable route reachability probe in any
+case.
+
+Signed-off-by: Stefan Agner <[email protected]>
+---
+ net/ipv6/Kconfig | 9 +++++++++
+ net/ipv6/route.c | 3 ++-
+ 2 files changed, 11 insertions(+), 1 deletion(-)
+
+diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
+index 08d4b7132d4c..242bf2eeb7ae 100644
+--- a/net/ipv6/Kconfig
++++ b/net/ipv6/Kconfig
+@@ -48,6 +48,15 @@ config IPV6_OPTIMISTIC_DAD
+
+ 	  If unsure, say N.
+
++config IPV6_REACHABILITY_PROBE
++	bool "IPv6: Always use reachability probing (RFC 4191)"
++	help
++	  By default reachability probing is disabled on router devices (when
++	  IPv6 forwarding is enabled). This option explicitly enables
++	  reachability probing always.
++
++	  If unsure, say N.
++
+ config INET6_AH
+ 	tristate "IPv6: AH transformation"
+ 	select XFRM_AH
+diff --git a/net/ipv6/route.c b/net/ipv6/route.c
+index 56525b5b95a2..916769b9a772 100644
+--- a/net/ipv6/route.c
++++ b/net/ipv6/route.c
+@@ -2211,7 +2211,8 @@ struct rt6_info *ip6_pol_route(struct net *net, struct fib6_table *table,
+
+ 	strict |= flags & RT6_LOOKUP_F_IFACE;
+ 	strict |= flags & RT6_LOOKUP_F_IGNORE_LINKSTATE;
+-	if (net->ipv6.devconf_all->forwarding == 0)
++	if (net->ipv6.devconf_all->forwarding == 0 ||
++	    IS_ENABLED(CONFIG_IPV6_REACHABILITY_PROBE))
+ 		strict |= RT6_LOOKUP_F_REACHABLE;
+
+ 	rcu_read_lock();