kcm: fix strp_init() order and cleanup

strp_init() is called just a few lines above this csk->sk_user_data
check, it also initializes strp->work etc., therefore, it is
unnecessary to call strp_done() to cancel the freshly initialized
work.

And if sk_user_data is already used by KCM, psock->strp should not be
touched, particularly strp->work state, so we need to move strp_init()
after the csk->sk_user_data check.

This also makes a lockdep warning reported by syzbot go away.

Reported-and-tested-by: [e-mail]
Reported-by: [e-mail]
Fixes: e5571240236c ("kcm: Check if sk_user_data already set in kcm_attach")
Fixes: dff8baa26117 ("kcm: Call strp_stop before strp_done in kcm_attach")
Cc: Tom Herbert <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/kcm/kcmsock.c

···
 	psock->sk = csk;
 	psock->bpf_prog = prog;
 
-	err = strp_init(&psock->strp, csk, &cb);
-	if (err) {
-		kmem_cache_free(kcm_psockp, psock);
-		goto out;
-	}
-
 	write_lock_bh(&csk->sk_callback_lock);
 
 	/* Check if sk_user_data is already by KCM or someone else.
···
 	 */
 	if (csk->sk_user_data) {
 		write_unlock_bh(&csk->sk_callback_lock);
-		strp_stop(&psock->strp);
-		strp_done(&psock->strp);
 		kmem_cache_free(kcm_psockp, psock);
 		err = -EALREADY;
+		goto out;
+	}
+
+	err = strp_init(&psock->strp, csk, &cb);
+	if (err) {
+		write_unlock_bh(&csk->sk_callback_lock);
+		kmem_cache_free(kcm_psockp, psock);
 		goto out;
 	}