Allow 0.0.0.0/8 as a valid address range

The longstanding prohibition against using 0.0.0.0/8 dates back
to two issues with the early internet.

There was an interoperability problem with BSD 4.2 in 1984, fixed in
BSD 4.3 in 1986. BSD 4.2 has long since been retired.

Secondly, addresses of the form 0.x.y.z were initially defined only as
a source address in an ICMP datagram, indicating "node number x.y.z on
this IPv4 network", by nodes that know their address on their local
network, but do not yet know their network prefix, in RFC0792 (page
19).  This usage of 0.x.y.z was later repealed in RFC1122 (section
3.2.2.7), because the original ICMP-based mechanism for learning the
network prefix was unworkable on many networks such as Ethernet (which
have longer addresses that would not fit into the 24 "node number"
bits).  Modern networks use reverse ARP (RFC0903) or BOOTP (RFC0951)
or DHCP (RFC2131) to find their full 32-bit address and CIDR netmask
(and other parameters such as default gateways). 0.x.y.z has had
16,777,215 addresses in 0.0.0.0/8 space left unused and reserved for
future use, since 1989.

This patch allows for these 16m new IPv4 addresses to appear within
a box or on the wire. Layer 2 switches don't care.

0.0.0.0/32 is still prohibited, of course.

Signed-off-by: Dave Taht <dave.taht@gmail.com>
Signed-off-by: John Gilmore <gnu@toad.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>