Previously, in the AF_INET6 case, inet_net_pton() would only copy out
the part of the address covered by the provided prefix length. For
example, if the input address was "2001:db8::1/32", it would only
copy out 4 bytes of the address.

This causes two problems. Firstly, this means the caller has to zero
the provided buffer before calling inet_net_pton(), which is neither
expected nor documented in the manual page.

Secondly, it means that with an input like "2001:db8::1/32", the
last bit of the address would not be copied out, so the result would
be "2001:db8::" instead of "2001:db8::1". This differs from the IPv4
behaviour, where the entire address is always copied out.

Fix this by always copying out the full address, and adjust the tests
to handle this.

MFC after: 3 days