fib_tests: Add tests for invalid metric on route

Add ipv4 and ipv6 test cases with an invalid metrics option causing
ip_metrics_convert to fail. Tests clean up path during route add.

Also, add nodad to to ipv6 address add. When running ipv6_route_metrics
directly seeing an occasional failure on the "Using route with mtu metric"
test case.

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