ipv6: del unreachable route when an addr is deleted on lo

When an address is added on loopback (ip -6 a a 2002::1/128 dev lo), two routes
are added:
 - one in the local table:
    local 2002::1 via :: dev lo  proto none  metric 0
 - one the in main table (for the prefix):
    unreachable 2002::1 dev lo  proto kernel  metric 256  error -101

When the address is deleted, the route inserted in the main table remains
because we use rt6_lookup(), which returns NULL when dst->error is set, which
is the case here! Thus, it is better to use ip6_route_lookup() to avoid this
kind of filter.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>