Traditionally we had 2 sources of information whether the added/delete route request targets network or a host route:
netmask (`RTA_NETMASK`) and `RTF_HOST` flag.
The former one is tricky: netmask can be empty or can explicitly specify the host netmask. Parsing netmask sockaddr requires per-family parsing and that's what rtsock code traditionally avoided. As a result, consistency was not enforced and it was possible to specify network with the `RTF_HOST` flag and vice versa.
Continue normalization efforts from D29826 and D29826 and ensure that `RTF_HOST` flag always reflects host/network data from netmask field.