ospf6d: fix logging of border router routes

ospf6d: fix logging of border router routes

The prefix of routes to border routers consists of two pieces of
information embedded in a single struct (prefix.u.lp):

  struct prefix {
          uint8_t family;
          uint16_t prefixlen;
          union {
  		[snip]
                  struct {
                          struct in_addr id;
                          struct in_addr adv_router;
                  } lp;
          } u __attribute__((aligned(8)));
  };

As such, using prefix2str() (or the %pFX format specifier) isn't
correct when logging such routes.

This commit adds a few special cases here and there to handle
OSPF6_DEST_TYPE_ROUTER routes differently. It'd probably be a good
idea to add a helper function to handle all cases in a single place,
but that can be left for a second moment.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>