fib_tests: Add tests for metrics on routes

Add ipv4 and ipv6 test cases for metrics (mtu) when fib entries are
created. Can be used with kmemleak to see leaks with both fib entries
and dst_entry.

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