net/ipv6: need update peer route when modify metric

When we modify the route metric, the peer address's route need also
be updated. Before the fix:

+ ip addr add dev dummy1 2001:db8::1 peer 2001:db8::2 metric 60
+ ip -6 route show dev dummy1
2001:db8::1 proto kernel metric 60 pref medium
2001:db8::2 proto kernel metric 60 pref medium
+ ip addr change dev dummy1 2001:db8::1 peer 2001:db8::2 metric 61
+ ip -6 route show dev dummy1
2001:db8::1 proto kernel metric 61 pref medium
2001:db8::2 proto kernel metric 60 pref medium

After the fix:
+ ip addr change dev dummy1 2001:db8::1 peer 2001:db8::2 metric 61
+ ip -6 route show dev dummy1
2001:db8::1 proto kernel metric 61 pref medium
2001:db8::2 proto kernel metric 61 pref medium

Fixes: 8308f3ff ("net/ipv6: Add support for specifying metric of connected routes")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>