Set the back pointer from the prefix to the interface before adding
the prefix to the interface's prefix list.  This shouldn't make a
difference, since rtadvd(8) is single-threaded, but I've seen it crash
in delete_prefix() with pfx_rainfo == NULL, and this is the only place
where a prefix can be added to the list with a NULL pfx_rainfo.

MFC after:	3 days