bpf: Stop the psock parser before canceling its work

We might have never enabled (started) the psock's parser, in which case it
will not get stopped when destroying the psock. This leads to a warning
when trying to cancel parser's work from psock's deferred destructor:

[  405.325769] WARNING: CPU: 1 PID: 3216 at net/strparser/strparser.c:526 strp_done+0x3c/0x40
[  405.326712] Modules linked in: [last unloaded: test_bpf]
[  405.327359] CPU: 1 PID: 3216 Comm: kworker/1:164 Tainted: G        W         5.0.0 #42
[  405.328294] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
[  405.329712] Workqueue: events sk_psock_destroy_deferred
[  405.330254] RIP: 0010:strp_done+0x3c/0x40
[  405.330706] Code: 28 e8 b8 d5 6b ff 48 8d bb 80 00 00 00 e8 9c d5 6b ff 48 8b 7b 18 48 85 ff 74 0d e8 1e a5 e8 ff 48 c7 43 18 00 00 00 00 5b c3 <0f> 0b eb cf 66 66 66 66 90 55 89 f5 53 48 89 fb 48 83 c7 28 e8 0b
[  405.332862] RSP: 0018:ffffc900026bbe50 EFLAGS: 00010246
[  405.333482] RAX: ffffffff819323e0 RBX: ffff88812cb83640 RCX: ffff88812cb829e8
[  405.334228] RDX: 0000000000000001 RSI: ffff88812cb837e8 RDI: ffff88812cb83640
[  405.335366] RBP: ffff88813fd22680 R08: 0000000000000000 R09: 000073746e657665
[  405.336472] R10: 8080808080808080 R11: 0000000000000001 R12: ffff88812cb83600
[  405.337760] R13: 0000000000000000 R14: ffff88811f401780 R15: ffff88812cb837e8
[  405.338777] FS:  0000000000000000(0000) GS:ffff88813fd00000(0000) knlGS:0000000000000000
[  405.339903] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  405.340821] CR2: 00007fb11489a6b8 CR3: 000000012d4d6000 CR4: 00000000000406e0
[  405.341981] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  405.343131] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  405.344415] Call Trace:
[  405.344821]  sk_psock_destroy_deferred+0x23/0x1b0
[  405.345585]  process_one_work+0x1ae/0x3e0
[  405.346110]  worker_thread+0x3c/0x3b0
[  405.346576]  ? pwq_unbound_release_workfn+0xd0/0xd0
[  405.347187]  kthread+0x11d/0x140
[  405.347601]  ? __kthread_parkme+0x80/0x80
[  405.348108]  ret_from_fork+0x35/0x40
[  405.348566] ---[ end trace a4a3af4026a327d4 ]---

Stop psock's parser just before canceling its work.

Fixes: 1d79895a ("sk_msg: Always cancel strp work before freeing the psock")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/skmsg.c

@@ -554,6 +554,7 @@ static void sk_psock_destroy_deferred(struct work_struct *gc)
 	struct sk_psock *psock = container_of(gc, struct sk_psock, gc);
 
 	/* No sk_callback_lock since already detached. */
+	strp_stop(&psock->parser.strp);
 	strp_done(&psock->parser.strp);
 
 	cancel_work_sync(&psock->work);