ipv6: fix route lookup in addrconf_prefix_rcv()

The route lookup to find a previously auto-configured route for a prefixes used
to use rt6_lookup(), with the prefix from the RA used as an address. However,
that kind of lookup ignores routing tables, the prefix length and route flags,
so when there were other matching routes, even in different tables and/or with
a different prefix length, the wrong route would be manipulated.

Now, a new function "addrconf_get_prefix_route()" is used for the route lookup,
which searches in RT6_TABLE_PREFIX and takes the prefix-length and route flags
into account.

Signed-off-by: Andreas Hofmeister <andi@collax.com>
Signed-off-by: David S. Miller <davem@davemloft.net>