net: ipv6: lower ndisc notifier priority below addrconf

ndisc_notify is used to send unsolicited neighbor advertisements
(e.g., on a link up). Currently, the ndisc notifier is run before the
addrconf notifer which means NA's are not sent for link-local addresses
which are added by the addrconf notifier.

Fix by lowering the priority of the ndisc notifier. Setting the priority
to ADDRCONF_NOTIFY_PRIORITY - 5 means it runs after addrconf and before
the route notifier which is ADDRCONF_NOTIFY_PRIORITY - 10.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>