selftests: fib_tests: Add ipv6 route add append replace tests

Add IPv6 route tests covering add, append and replace permutations.
Assumes the ability to add a basic single path route works; this is
required for example when adding an address to an interface.

$ fib_tests.sh -t ipv6_rt

IPv6 route add / append tests
    TEST: Attempt to add duplicate route - gw                           [ OK ]
    TEST: Attempt to add duplicate route - dev only                     [ OK ]
    TEST: Attempt to add duplicate route - reject route                 [ OK ]
    TEST: Add new route for existing prefix (w/o NLM_F_EXCL)            [ OK ]
    TEST: Append nexthop to existing route - gw                         [ OK ]
    TEST: Append nexthop to existing route - dev only                   [ OK ]
    TEST: Append nexthop to existing route - reject route               [ OK ]
    TEST: Append nexthop to existing reject route - gw                  [ OK ]
    TEST: Append nexthop to existing reject route - dev only            [ OK ]
    TEST: Add multipath route                                           [ OK ]
    TEST: Attempt to add duplicate multipath route                      [ OK ]
    TEST: Route add with different metrics                              [ OK ]
    TEST: Route delete with metric                                      [ OK ]

IPv6 route replace tests
    TEST: Single path with single path                                  [ OK ]
    TEST: Single path with multipath                                    [ OK ]
    TEST: Single path with reject route                                 [ OK ]
    TEST: Single path with single path via multipath attribute          [ OK ]
    TEST: Invalid nexthop                                               [ OK ]
    TEST: Single path - replace of non-existent route                   [ OK ]
    TEST: Multipath with multipath                                      [ OK ]
    TEST: Multipath with single path                                    [ OK ]
    TEST: Multipath with single path via multipath attribute            [ OK ]
    TEST: Multipath with reject route                                   [ OK ]
    TEST: Multipath - invalid first nexthop                             [ OK ]
    TEST: Multipath - invalid second nexthop                            [ OK ]
    TEST: Multipath - replace of non-existent route                     [ OK ]
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>