IPv6: set RTPROT_KERNEL to initial route

The use of unspecified protocol in IPv6 initial route prevents quagga to
install IPv6 default route:
# show ipv6 route
S   ::/0 [1/0] via fe80::1, eth1_0
K>* ::/0 is directly connected, lo, rej
C>* ::1/128 is directly connected, lo
C>* fe80::/64 is directly connected, eth1_0

# ip -6 route
fe80::/64 dev eth1_0  proto kernel  metric 256  mtu 1500 advmss 1440
hoplimit -1
ff00::/8 dev eth1_0  metric 256  mtu 1500 advmss 1440 hoplimit -1
unreachable default dev lo  proto none  metric -1  error -101 hoplimit 255

The attached patch ensures RTPROT_KERNEL to the default initial route
and fixes the problem for quagga.
This is similar to "ipv6: protocol for address routes"
f410a1fb.

# show ipv6 route
S>* ::/0 [1/0] via fe80::1, eth1_0
C>* ::1/128 is directly connected, lo
C>* fe80::/64 is directly connected, eth1_0

# ip -6 route
fe80::/64 dev eth1_0  proto kernel  metric 256  mtu 1500 advmss 1440
hoplimit -1
fe80::/64 dev eth1_0  proto kernel  metric 256  mtu 1500 advmss 1440
hoplimit -1
ff00::/8 dev eth1_0  metric 256  mtu 1500 advmss 1440 hoplimit -1
default via fe80::1 dev eth1_0  proto zebra  metric 1024  mtu 1500
advmss 1440 hoplimit -1
unreachable default dev lo  proto kernel  metric -1  error -101 hoplimit 255

Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>