ipv6: Pass fib6_result to rt6_find_cached_rt

Simplify rt6_find_cached_rt for the fast path cases and pass fib6_result
to rt6_find_cached_rt. Rename the local return variable to ret to maintain
consisting with fib6_result name.

Update the comment in rt6_find_cached_rt to reference the new names in
a fib6_info vs the old name when fib entries were an rt6_info.

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