ip: Properly display AF_BRIDGE address information for neighbor events

ip: Properly display AF_BRIDGE address information for neighbor events

The vxlan driver when a neighbor add/delete event occurs sends
NDA_DST filled with a union:

union vxlan_addr {
	struct sockaddr_in sin;
	struct sockaddr_in6 sin6;
	struct sockaddr sa;
};

This eventually calls rt_addr_n2a_r which had no handler for the
AF_BRIDGE family and "???" was being printed.

Add code to properly display this data when requested.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>