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

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>