ipv6: fix the noflags test in addrconf_get_prefix_route

The tests on the flags in addrconf_get_prefix_route() does no make
much sense: the 'noflags' parameter contains the set of flags that
must not match with the route flags, so the test must be done
against 'noflags', and not against 'flags'.

Signed-off-by: Romain Kuntz <r.kuntz@ipflavors.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>