db, IP2BIN: correct `format_ipv6()` output buffer sizes

db, IP2BIN: correct `format_ipv6()` output buffer sizes

`format_ipv6()` formats IPv6 addresses as hex-strings.  However, sizing for the
output buffer is not done quite right.

The elements of the `ipbin_array` array in ulogd_filter_IP2BIN.c are sized using
a local macro, `IPADDR_LENGTH`, which is defined as 128, the number of bits in
an IPv6 address; this is much larger than necessary.

Define an appropriate macro and use that instead.

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