ipv6: send NEWLINK on RA managed/otherconf changes

The kernel is applying the RA managed/otherconf flags silently and
forgets to send ifinfo notify to inform about their change when the
router provides a zero reachable_time and retrans_timer as dnsmasq
and many routers send it, which just means unspecified by this router
and the host should continue using whatever value it is already using.
Userspace may monitor the ifinfo notifications to activate dhcpv6.

Signed-off-by: Marius Tomaschewski <mt@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>