ipv6: fib6_rule_action_alt needs to return -EAGAIN

fib rule actions should return -EAGAIN for the rules to continue to the
next one. A recent change overwrote err with the lookup always returning
0 (future change will make it more like IPv4) which means the rules
stopped at the first (e.g., local table lookup only). Catch and reset err
to -EAGAIN.

Fixes: effda4dd ("ipv6: Pass fib6_result to fib lookups")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>