network: IPv6 Compliance RFC4862: Address Lifetime Expiry (Hosts Only) [v6LC.3.2.2]

network: IPv6 Compliance RFC4862: Address Lifetime Expiry (Hosts Only) [v6LC.3.2.2]

RFC 4862 Section 5.5.3, bullet e, sub-bullet 3 applies to existing
addresses, i.e. when address_get() returns success. If the address is
new (i.e. address_get() fails), then we should not be adding 2 hours to
the lifetime_valid_usec. Instead, use the valid_lifetime from the RA's
Prefix Information Option.

This change allows v6LC.3.2.2 to pass. Also verified all v6LC3.2.* tests
pass. This covers all the v6LC tests from Group2: Router Advertisement
Processing and Address Lifetime.

Fixes #32652.