drop_monitor: synchronize tracepoint unregistration on error path

If register_trace_napi_poll() fails in net_dm_trace_on_set(),
unregister_trace_kfree_skb() is called to roll back the kfree_skb
tracepoint registration.

However, tracepoint_synchronize_unregister() is omitted before calling
cancel_work_sync() and module_put(). An in-flight probe executing
concurrently on another CPU could call schedule_work() after
cancel_work_sync() has already returned, leaving a pending work item
scheduled after the module reference is dropped. If the module is then
unloaded, executing the work item triggers a kernel panic.

Add tracepoint_synchronize_unregister() after unregister_trace_kfree_skb()
in the error path, matching net_dm_trace_off_set() and
net_dm_hw_probe_unregister().

Fixes: 7c747838a558 ("drop_monitor: Split tracing enable / disable to different functions")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Link: https://patch.msgid.link/20260910204612.3762015-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>