act_ife: only acquire tcf_lock for existing actions

Alexey reported that we have GFP_KERNEL allocation when
holding the spinlock tcf_lock. Actually we don't have
to take that spinlock for all the cases, especially
for the new one we just create. To modify the existing
actions, we still need this spinlock to make sure
the whole update is atomic.

For net-next, we can get rid of this spinlock because
we already hold the RTNL lock on slow path, and on fast
path we can use RCU to protect the metalist.

Joint work with Jamal.

Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
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>