Don't bind loopback to all IPs in an IPv6 subnet

An earlier change considered the loopback bound to all addresses in an
assigned subnet. This should have only be done for IPv4 to maintain
compatability with Linux:

```
$ ip addr show dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group ...
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
$ ping 2001:db8::1
PING 2001:db8::1(2001:db8::1) 56 data bytes
^C
--- 2001:db8::1 ping statistics ---
4 packets transmitted, 0 received, 100% packet loss, time 3062ms

$ ping 2001:db8::2
PING 2001:db8::2(2001:db8::2) 56 data bytes
^C
--- 2001:db8::2 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2030ms

$ sudo ip addr add 2001:db8::1/64 dev lo
$ ping 2001:db8::1
PING 2001:db8::1(2001:db8::1) 56 data bytes
64 bytes from 2001:db8::1: icmp_seq=1 ttl=64 time=0.055 ms
64 bytes from 2001:db8::1: icmp_seq=2 ttl=64 time=0.074 ms
64 bytes from 2001:db8::1: icmp_seq=3 ttl=64 time=0.073 ms
64 bytes from 2001:db8::1: icmp_seq=4 ttl=64 time=0.071 ms
^C
--- 2001:db8::1 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3075ms
rtt min/avg/max/mdev = 0.055/0.068/0.074/0.007 ms
$ ping 2001:db8::2
PING 2001:db8::2(2001:db8::2) 56 data bytes
From 2001:db8::1 icmp_seq=1 Destination unreachable: No route
From 2001:db8::1 icmp_seq=2 Destination unreachable: No route
From 2001:db8::1 icmp_seq=3 Destination unreachable: No route
From 2001:db8::1 icmp_seq=4 Destination unreachable: No route
^C
--- 2001:db8::2 ping statistics ---
4 packets transmitted, 0 received, +4 errors, 100% packet loss, time 3070ms
```

Test: integration_test.TestLoopbackAcceptAllInSubnet
PiperOrigin-RevId: 329011566

0 files changed, 0 insertions, 0 deletions