ipv6: Use result arg in fib_lookup_arg consistently

arg.result is sometimes used as fib6_result and sometimes used to
hold the rt6_info. Add rt6_info to fib6_result and make the use
of arg.result consistent through ipv6 rules.

The rt6 entry is filled in for lookups returning a dst_entry, but not
for direct fib_lookups that just want a fib6_info.

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>