ipv6: Pass fib6_result to rt6_insert_exception

Update rt6_insert_exception to take a fib6_result over a fib6_info.
Change ort to f6i from the fib6_result and rename to better reflect
what it references (a fib6_info).

Since this function is already getting changed, update the comments
to reference fib6_info variables rather than the older rt6_info.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>