nexthops: Add selftests for cleanup of known bad route add

Test cleanup path for routes usinig nexthop objects before the
reference is taken on the nexthop. Specifically, bad metric for
ipv4 and ipv6 and source routing for ipv6.

Selftests that correspond to the recent bug fix:
    821bbf79 ("ipv6: Fix KASAN: slab-out-of-bounds Read in fib6_nh_flush_exceptions")

Signed-off-by: David Ahern <dsahern@kernel.org>
Cc: Coco Li <lixiaoyan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>