dhcpv6: Add error checking to handle renew failure for IA_PD operations

dhcpv6: Add error checking to handle renew failure for IA_PD operations

Check for error status code in the IA Prefix option in replies to RENEW
messages.

This fixes a problem where odhcp6c thinks that renewal succeeded, when
actually the upstream router is no longer routing this prefix for us.

See https://github.com/openwrt/odhcp6c/issues/61#issuecomment-2509536512

Signed-off-by: Luke Deller <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/103
Signed-off-by: Álvaro Fernández Rojas <[email protected]>