[IPV6] ROUTE: Fix looking up a route on subtree.

Even on RTN_ROOT node, we need to process its subtree first.
Fix NULL pointer dereference in fib6_locate().

Based on MIPL2 kernel patch.

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