ipv4: provide addr and netconf dump consistency info

This patch takes benefit of dev_addr_genid and 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 seq and prev_seq must be reset between each family in rtnl_dump_all()
because they are specific to each family.

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>