ipv4/fib: use synchronize_net() when holding RTNL

tnode_free() should use synchronize_net()
instead of syncronize_rcu() to release RTNL sooner.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>