net: sockets: getnameinfo: Cast to sockaddr_in6 instead of sockaddr_in

As explained in the comment in the code, both structs have the same
offsets for it's fields, but sockaddr_in is smaller, hence it can
confuse static analyzer, giving warnings about potential out-of-bound
access. Therefore, cast to sockaddr_in6 instead to avoid the warning.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>