ipv6: Refactor find_rr_leaf

find_rr_leaf has 3 loops over fib_entries calling find_match. The loops
are very similar with differences in start point and whether the metric
is evaluated:
    1. start at rr_head, no extra loop compare, check fib metric
    2. start at leaf, compare rt against rr_head, check metric
    3. start at cont (potential saved point from earlier loops), no
       extra loop compare, no metric check

Create 1 loop that is called 3 different times. This will make a
later change with multipath nexthop objects much simpler.

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