ipv6: provide addr and netconf dump consistency info

This patch adds a dev_addr_genid for IPv6. The goal is to use it, combined with
dev_base_seq to check if a change occurs during a netlink dump.
If a change is detected, the flag NLM_F_DUMP_INTR is set in the first message
after the dump was interrupted.

Note that only dump of unicast addresses is checked (multicast and anycast are
not checked).

Reported-by: Junwei Zhang <junwei.zhang@6wind.com>
Reported-by: Hongjun Li <hongjun.li@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>