ipv6cp: Fix enforcing peer IPv6 interface identifier (#281)

ipv6cp: Fix enforcing peer IPv6 interface identifier (#281)

Enforcing peer IPv6 interface identifier is broken in the same way as it
was broken for peer IPv4 address prior commit 9fe8923419a9 ("pppd: Fix
enforcing peer IP address").

IPv6 test case:

    ./pppd noauth asyncmap 0 default-asyncmap novj noaccomp nopcomp nodeflate nobsdcomp nomagic  local nolock nodetach  noip  ipv6 ::2,::1  debug pty "./pppd noauth asyncmap 0 default-asyncmap novj noaccomp nopcomp nodeflate nobsdcomp nomagic  local nolock nodetach  noip  ipv6 ::1:1,::1:2  notty"

Without this patch output from first pppd is:

    sent [IPV6CP ConfReq id=0x5 <addr fe80::0000:0000:0000:0002>]
    rcvd [IPV6CP ConfReq id=0x5 <addr fe80::0000:0000:0001:0001>]
    sent [IPV6CP ConfNak id=0x5 <addr fe80::0000:0000:0000:0001>]
    rcvd [IPV6CP ConfNak id=0x5 <addr fe80::0000:0000:0001:0002>]
    sent [IPV6CP ConfReq id=0x6]
    rcvd [IPV6CP ConfReq id=0x6]
    sent [IPV6CP ConfAck id=0x6]
    rcvd [IPV6CP ConfAck id=0x6]
    local  LL address fe80::0000:0000:0000:0002
    remote LL address fe80::0000:0000:0000:0001

And system configured IPv6 addresses as:

    124: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 3
        link/ppp
        inet6 fe80::2 peer fe80::1/128 scope link nodad
           valid_lft forever preferred_lft forever
    125: ppp1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 3
        link/ppp
        inet6 fe80::1:1 peer fe80::1:2/128 scope link nodad
           valid_lft forever preferred_lft forever

So both pppd use different IPv6 LL addresses and therefore IPv6 connection
is totally broken between these pppd instances.

With this patch output from first pppd is:

    sent [IPV6CP ConfReq id=0x5 <addr fe80::0000:0000:0000:0002>]
    rcvd [IPV6CP ConfReq id=0x5 <addr fe80::0000:0000:0001:0001>]
    sent [IPV6CP ConfNak id=0x5 <addr fe80::0000:0000:0000:0001>]
    rcvd [IPV6CP ConfNak id=0x5 <addr fe80::0000:0000:0001:0002>]
    sent [IPV6CP ConfReq id=0x6]
    rcvd [IPV6CP ConfReq id=0x6]
    sent [IPV6CP ConfAck id=0x6]
    rcvd [IPV6CP ConfAck id=0x6]
    Peer refused to agree to his interface identifier
    sent [IPV6CP TermReq id=0x7 "Refused his interface identifier"]
    rcvd [IPV6CP TermReq id=0x7 "Refused his interface identifier"]
    sent [IPV6CP TermAck id=0x7]
    rcvd [IPV6CP TermAck id=0x7]
    sent [LCP TermReq id=0x2 "No network protocols running"]
    rcvd [LCP TermAck id=0x2]
    Connection terminated.
    Connect time 0.0 minutes.
    Sent 148 bytes, received 148 bytes.

Now pppd correctly detected that IPv6 interface identifiers negotiation
failed and closed IPv6 connection.

Signed-off-by: Pali Rohár <pali@kernel.org>