net: dns: Fix DNS caching bug

The current DNS caching logic is not aware of the IP address version.
If there is a cached address for a query, the caller of
dns_get_addr_info() will receive that address, even if it is not the
same version as the caller requested. For example:

- dns_get_addr_info() is called to resolve an IPv4 address
- the DNS client caches the IPv4 address that was received
- dns_get_addr_info() is called to resolve an IPv6 address
- the DNS client sees that there is a cached IPv4 address for the
  given query
- the DNS client does not check that the cached address is of the
  requested version (IPv6)
- the cached IPv4 address is returned from dns_get_addr_info()

This changes the DNS client to check IP address version when searching
cached addresses.

Signed-off-by: Noah Olson <noah@wavelynx.com>