IP2STR: correct address buffer size

IP2STR: correct address buffer size

The elements of the `ipstr_array` array are `IPADDR_LENGTH` bytes long where
`IPADDR_LENGTH` is a local macro defined as 128.  However, this is the number of
bits in an IPv6 address, but the elements of `ipstr_array` only need to be big
enough to be used for the output of `inet_ntop`.  Use the standard
`INET6_ADDRSTRLEN` macro instead.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>