Not all possible DNS names will survive serialization. Restrict the set
of valid dns names to LDH encoded names.
Fixes: 25c33e3500 (network: parse RFC9463 DHCPv4 DNR option, 2024-01-16)
Fixes: a07e83cc58 (network: Parse RFC9463 DHCPv6 DNR option, 2024-01-17)
Fixes: 0c90d1d2f2 (ndisc: Parse RFC9463 encrypted DNS (DNR) option, 2024-01-19)