net_sched: check NULL on error path in route4_change()

net_sched: check NULL on error path in route4_change()

On error path in route4_change(), 'f' could be NULL,
so we should check NULL before calling tcf_exts_destroy().

Fixes: 224fdce15787 ("net_sched: properly handle failure case of tcf_exts_init()")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>