net: fib: Decrease one unnecessary rt cache flush in fib_disable_ip

The func fib_flush already flushes the rt cache if necessary, so it
is not necessary to invoke rt_cache_flush again in fib_disable_ip.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>