ipv6: More buffers in iaddr_to_string

We are going to have addresses of multiple address families in various
places, which will mean more calls to iaddr_to_string for the benefit
of the same logging statement.

Increase the number of static buffers used by iaddr_to_string from 2
to 8.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>