[IPV6]: Make sure fib6_rule_lookup doesn't return NULL

The callers of fib6_rule_lookup don't expect it to return NULL,
therefore it must return ip6_null_entry whenever fib_rule_lookup fails.

Signed-off-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>