[IPV6] ROUTE: Unify RT6_F_xxx and RT6_SELECT_F_xxx flags

Unify RT6_F_xxx and RT6_SELECT_F_xxx flags into
RT6_LOOKUP_F_xxx flags, and put them into ip6_route.h

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>