network: directly compare with in_addr element for IPv4 case

When peer address is set, address_compare() (or address_equal()) mostly
does not work in link_is_static_address_configured(), as an Address object
stored in a Link does not contain peer addresses. So, we need to also
compare with in_addr element for IPv4 case.

Fixes #17304.

(Based on 9b9c5fff)