If flower filter is created without the skip_sw flag, fl_mask_put()
can race with fl_classify() and we can destroy the mask rhashtable
while a lookup operation is accessing it.

 BUG: unable to handle kernel paging request at 00000000000911d1
 PGD 0 P4D 0
 SMP PTI
 CPU: 3 PID: 5582 Comm: vhost-5541 Not tainted 4.18.0-rc1.vanilla+ #1950
 Hardware name: Dell Inc. PowerEdge R730/072T6D, BIOS 2.1.7 06/16/2016
 RIP: 0010:rht_bucket_nested+0x20/0x60
 Code: 31 c8 c1 c1 18 29 c8 c3 66 90 8b 4f 04 ba 01 00 00 00 8b 07 48 8b bf 80 00 00 0
 RSP: 0018:ffffafc5cfbb7a48 EFLAGS: 00010206
 RAX: 0000000000001978 RBX: ffff9f12dff88a00 RCX: 00000000ffff9f12
 RDX: 00000000000911d1 RSI: 0000000000000148 RDI: 0000000000000001
 RBP: ffff9f12dff88a00 R08: 000000005f1cc119 R09: 00000000a715fae2
 R10: ffffafc5cfbb7aa8 R11: ffff9f1cb4be804e R12: ffff9f1265e13000
 R13: 0000000000000000 R14: ffffafc5cfbb7b48 R15: ffff9f12dff88b68
 FS:  0000000000000000(0000) GS:ffff9f1d3f0c0000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00000000000911d1 CR3: 0000001575a94006 CR4: 00000000001626e0
 Call Trace:
  fl_lookup+0x134/0x140 [cls_flower]
  fl_classify+0xf3/0x180 [cls_flower]
  tcf_classify+0x78/0x150
  __netif_receive_skb_core+0x69e/0xa50
  netif_receive_skb_internal+0x42/0xf0
  tun_get_user+0xdd5/0xfd0 [tun]
  tun_sendmsg+0x52/0x70 [tun]
  handle_tx+0x2b3/0x5f0 [vhost_net]
  vhost_worker+0xab/0x100 [vhost]
  kthread+0xf8/0x130
  ret_from_fork+0x35/0x40
 Modules linked in: act_mirred act_gact cls_flower vhost_net vhost tap sch_ingress
 CR2: 00000000000911d1

Fix the above waiting for a RCU grace period before destroying the
rhashtable: we need to use tcf_queue_work(), as rhashtable_destroy()
must run in process context, as pointed out by Cong Wang.

v1 -> v2: use tcf_queue_work to run rhashtable_destroy().

Fixes: 05cd271f ("cls_flower: Support multiple masks per priority")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>