nla_memdup_cookie was overwriting err value, declared at function
scope and earlier initialized with result of ->init(). At success
nla_memdup_cookie() returns 0, and thus module refcnt decremented,
although the action was installed.

$ sudo tc actions add action pass index 1 cookie 1234
$ sudo tc actions ls action gact

        action order 0: gact action pass
         random type none pass val 0
         index 1 ref 1 bind 0
$
$ lsmod
Module                  Size  Used by
act_gact               16384  0
...
$
$ sudo rmmod act_gact
[   52.310283] ------------[ cut here ]------------
[   52.312551] WARNING: CPU: 1 PID: 455 at kernel/module.c:1113
module_put+0x99/0xa0
[   52.316278] Modules linked in: act_gact(-) crct10dif_pclmul crc32_pclmul
ghash_clmulni_intel psmouse pcbc evbug aesni_intel aes_x86_64 crypto_simd
serio_raw glue_helper pcspkr cryptd
[   52.322285] CPU: 1 PID: 455 Comm: rmmod Not tainted 4.10.0+ #11
[   52.324261] Call Trace:
[   52.325132]  dump_stack+0x63/0x87
[   52.326236]  __warn+0xd1/0xf0
[   52.326260]  warn_slowpath_null+0x1d/0x20
[   52.326260]  module_put+0x99/0xa0
[   52.326260]  tcf_hashinfo_destroy+0x7f/0x90
[   52.326260]  gact_exit_net+0x27/0x40 [act_gact]
[   52.326260]  ops_exit_list.isra.6+0x38/0x60
[   52.326260]  unregister_pernet_operations+0x90/0xe0
[   52.326260]  unregister_pernet_subsys+0x21/0x30
[   52.326260]  tcf_unregister_action+0x68/0xa0
[   52.326260]  gact_cleanup_module+0x17/0xa0f [act_gact]
[   52.326260]  SyS_delete_module+0x1ba/0x220
[   52.326260]  entry_SYSCALL_64_fastpath+0x1e/0xad
[   52.326260] RIP: 0033:0x7f527ffae367
[   52.326260] RSP: 002b:00007ffeb402a598 EFLAGS: 00000202 ORIG_RAX:
00000000000000b0
[   52.326260] RAX: ffffffffffffffda RBX: 0000559b069912a0 RCX: 00007f527ffae367
[   52.326260] RDX: 000000000000000a RSI: 0000000000000800 RDI: 0000559b06991308
[   52.326260] RBP: 0000000000000003 R08: 00007f5280264420 R09: 00007ffeb4029511
[   52.326260] R10: 000000000000087b R11: 0000000000000202 R12: 00007ffeb4029580
[   52.326260] R13: 0000000000000000 R14: 0000000000000000 R15: 0000559b069912a0
[   52.354856] ---[ end trace 90d89401542b0db6 ]---
$

With the fix:

$ sudo modprobe act_gact
$ lsmod
Module                  Size  Used by
act_gact               16384  0
...
$ sudo tc actions add action pass index 1 cookie 1234
$ sudo tc actions ls action gact

        action order 0: gact action pass
         random type none pass val 0
         index 1 ref 1 bind 0
$
$ lsmod
Module                  Size  Used by
act_gact               16384  1
...
$ sudo rmmod act_gact
rmmod: ERROR: Module act_gact is in use
$
$ sudo /home/mrv/bin/tc actions del action gact index 1
$ sudo rmmod act_gact
$ lsmod
Module                  Size  Used by
$

Fixes: 1045ba77

 ("net sched actions: Add support for user cookies")
Signed-off-by: Roman Mashak <mrv@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>