subnet_to_string: Do not allocate

subnet_to_string: Do not allocate

None of the three call sites want to keep the value for any length of
time - they just use it right away.  Replace the allocation with a use
of the round-robin buffers from ipaddr_getbuf, and remove the frees at
the call sites.

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