net/sched: act_tunnel_key: fix memory leak in case of action replace

running the following TDC test cases:

 7afc - Replace tunnel_key set action with all parameters
 364d - Replace tunnel_key set action with all parameters and cookie

it's possible to trigger kmemleak warnings like:

  unreferenced object 0xffff94797127ab40 (size 192):
  comm "tc", pid 3248, jiffies 4300565293 (age 1006.862s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 c0 93 f9 8a ff ff ff ff  ................
    41 84 ee 89 ff ff ff ff 00 00 00 00 00 00 00 00  A...............
  backtrace:
    [<000000001e85b61c>] tunnel_key_init+0x31d/0x820 [act_tunnel_key]
    [<000000007f3f6ee7>] tcf_action_init_1+0x384/0x4c0
    [<00000000e89e3ded>] tcf_action_init+0x12b/0x1a0
    [<00000000c1c8c0f8>] tcf_action_add+0x73/0x170
    [<0000000095a9fc28>] tc_ctl_action+0x122/0x160
    [<000000004bebeac5>] rtnetlink_rcv_msg+0x263/0x2d0
    [<000000009fd862dd>] netlink_rcv_skb+0x4a/0x110
    [<00000000b55199e7>] netlink_unicast+0x1a0/0x250
    [<000000004996cd21>] netlink_sendmsg+0x2c1/0x3c0
    [<000000004d6a94b4>] sock_sendmsg+0x36/0x40
    [<000000005d9f0208>] ___sys_sendmsg+0x280/0x2f0
    [<00000000dec19023>] __sys_sendmsg+0x5e/0xa0
    [<000000004b82ac81>] do_syscall_64+0x5b/0x180
    [<00000000a0f1209a>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
    [<000000002926b2ab>] 0xffffffffffffffff

when the tunnel_key action is replaced, the kernel forgets to release the
dst metadata: ensure they are released by tunnel_key_init(), the same way
it's done in tunnel_key_release().

Fixes: d0f6dd8a ("net/sched: Introduce act_tunnel_key")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>