inet: gethostid: only accept v4 addresses

inet: gethostid: only accept v4 addresses

getaddrinfo() returns addresses from, at least, ip(7) and ipv6(7),
but _addr() always uses sin_addr from struct sockaddr_in;
we're saved from wild unsoundness (or incompatibility)
by virtue of struct sockaddr_in6 having an always-0 u32 sin6_flowinfo
at the same offset, so we end up returning 0 anyway,
but in a round-about and definitely unintended way

Instead, limit the request to AF_INET, and fall through to the end
early, returning the default id=0

Signed-off-by: Ahelenia Ziemiańska <[email protected]>