dhcpv6: RA - Fix preferred lifetime

[systemcrash]

Comments welcome.

Would appreciate input from @dedeckeh and @PolynomialDivision who seem most familiar with the RA bits.

Tested on 23.05.0

Example Data

Set preferred_lifetime to 7m in GUI (without setting ra_useleasetime)

The following are ICMP Options from ICMPv6 RA packets.

Before patch:

ICMPv6 Option (Prefix information : fd51:1c2a:8909::/64)
    Type: Prefix information (3)
    Length: 4 (32 bytes)
    Prefix Length: 64
    Flag: 0xc0, On-link flag(L), Autonomous address-configuration flag(A)
    Valid Lifetime: Infinity (4294967295)
    Preferred Lifetime: Infinity (4294967295)
    Reserved
    Prefix: fd51:1c2a:8909::

After patch:

ICMPv6 Option (Prefix information : fd51:1c2a:8909::/64)
    Type: Prefix information (3)
    Length: 4 (32 bytes)
    Prefix Length: 64
    Flag: 0xc0, On-link flag(L), Autonomous address-configuration flag(A)
    Valid Lifetime: Infinity (4294967295)
    Preferred Lifetime: 420
    Reserved
    Prefix: fd51:1c2a:8909::