ipv6 addrconf: extend ifa_flags to u32

There is no more space in u8 ifa_flags. So do what davem suffested and
add another netlink attr called IFA_FLAGS for carry more flags.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>