bgp: fix example code

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: fix best_path_change_handler for best_path_lost

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: fix ipv6 peering regression

Workaround. Needs to clean up get_peername and get_sockname usage.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>