xfrm: clear mode callbacks after failed mode setup

xfrm_state_gc_task can run long after a failed IPTFS state setup. In the
reproduced case, __xfrm_init_state() cached x->mode_cbs, IPTFS setup
returned -ENOMEM before publishing mode_data, and the temporary module
reference from xfrm_get_mode_cbs() was dropped immediately. The dead state
then kept x->mode_cbs until deferred GC ran after xfrm_iptfs had been
unloaded.

Clear x->mode_cbs when mode init or clone fails before publishing
mode_data. Those states never installed mode-specific state or the
long-term IPTFS module pin, so deferred GC has nothing mode-specific to
destroy and must not retain a callback table pointer past the temporary
lookup reference.

The buggy scenario involves two paths, with each column showing the order
within that path:

failed setup path:
1. cache x->mode_cbs
2. mode setup fails before mode_data
3. drop the temporary module ref
4. dead state keeps x->mode_cbs cached

GC/unload path:
1. xfrm_state_put() queues GC work
2. xfrm_iptfs unloads later
3. xfrm_state_gc_task runs
4. GC dereferences stale x->mode_cbs

This also covers the failed clone path where clone_state() returns before
publishing mode_data.

Validation reproduced this kernel report:
Kernel panic - not syncing: Fatal exception
CONFIG_FAULT_INJECTION_STACKTRACE_FILTER=y
failslab_stacktrace_filter matched xfrm_iptfs frames
ack_error=-12
FAULT_INJECTION: forcing a failure
BUG: unable to handle page fault
Workqueue: events xfrm_state_gc_task
RIP: xfrm_state_gc_task+0x142/0x650
Modules linked in: esp4_offload xfrm_user [last unloaded: xfrm_iptfs]
Kernel panic - not syncing: Fatal exception

Fixes: 4b3faf6 ("xfrm: iptfs: add new iptfs xfrm mode impl")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: Cen Zhang

net/xfrm/xfrm_state.c

@@ -2072,8 +2072,11 @@ static struct xfrm_state *xfrm_state_clone_and_setup(struct xfrm_state *orig,
 
 	x->mode_cbs = orig->mode_cbs;
 	if (x->mode_cbs && x->mode_cbs->clone_state) {
-		if (x->mode_cbs->clone_state(x, orig))
+		if (x->mode_cbs->clone_state(x, orig)) {
+			if (!x->mode_data)
+				x->mode_cbs = NULL;
 			goto error;
+		}
 	}
 
 	x->props.reqid = m->new_reqid;
@@ -3292,6 +3295,8 @@ int __xfrm_init_state(struct xfrm_state *x, struct netlink_ext_ack *extack)
 		if (x->mode_cbs->init_state)
 			err = x->mode_cbs->init_state(x);
 		module_put(x->mode_cbs->owner);
+		if (err && !x->mode_data)
+			x->mode_cbs = NULL;
 	}
 error:
 	return err;