generic: 5.10: backport Treat IPv4 segment's lowest address as unicast

Backport patch from kernel 5.14.

Treat only the highest, not the lowest, IPv4 address within a local
subnet as a broadcast address, as subnets do not need two different
broadcast addresses and networking documentation consistently prefers
the highest address as broadcast.

This patch was merged in upstream net-next tree in May 2021 at

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=94c821c74bf5

This eventually frees up one address per subnet. It matches behavior
suggested in our Internet-Draft, and also the default behavior of OpenBSD
and FreeBSD.

Signed-off-by: Seth David Schoen <schoen@loyalty.org>

target/linux/generic/backport-5.10/615-v5.14-ip-Treat-IPv4-segment-s-lowest-address-as-unicast.patch

@@ -0,0 +1,32 @@
+From 94c821c74bf5fe0c25e09df5334a16f98608db90 Mon Sep 17 00:00:00 2001
+From: Seth David Schoen <schoen@loyalty.org>
+Date: Wed, 12 May 2021 21:37:49 -0700
+Subject: [PATCH] ip: Treat IPv4 segment's lowest address as unicast
+
+Treat only the highest, not the lowest, IPv4 address within a local
+subnet as a broadcast address.
+
+Signed-off-by: Seth David Schoen <schoen@loyalty.org>
+Suggested-by: John Gilmore <gnu@toad.com>
+Acked-by: Dave Taht <dave.taht@gmail.com>
+Reviewed-by: David Ahern <dsahern@kernel.org>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+Link: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=94c821c74bf5
+---
+ net/ipv4/fib_frontend.c | 4 +---
+ 1 file changed, 1 insertion(+), 3 deletions(-)
+
+--- a/net/ipv4/fib_frontend.c
++++ b/net/ipv4/fib_frontend.c
+@@ -1129,10 +1129,8 @@ void fib_add_ifaddr(struct in_ifaddr *if
+ 				  prefix, ifa->ifa_prefixlen, prim,
+ 				  ifa->ifa_rt_priority);
+ 
+-		/* Add network specific broadcasts, when it takes a sense */
++		/* Add the network broadcast address, when it makes sense */
+ 		if (ifa->ifa_prefixlen < 31) {
+-			fib_magic(RTM_NEWROUTE, RTN_BROADCAST, prefix, 32,
+-				  prim, 0);
+ 			fib_magic(RTM_NEWROUTE, RTN_BROADCAST, prefix | ~mask,
+ 				  32, prim, 0);
+ 			arp_invalidate(dev, prefix | ~mask, false);