CVE-2026-72405: net: udp_tunnel: prevent double queueing in udp_tunnel_nic_device_sync

In the Linux kernel, the following vulnerability has been resolved:

net: udptunnel: prevent double queueing in udptunnelnicdevicesync

Yue Sun reported a use-after-free and debugobjects warning in udptunnelnicdevicesyncwork() during concurrent device operations.

The workqueue core clears the internal pending bit before invoking the worker. At that point, a concurrent thread can queue the work again. When the already running worker eventually clears the workpending flag to 0, it mistakenly clears the flag for the newly queued instance. udptunnelnicunregister() then observes workpending as 0 and frees the structure while the second work item is still active in the queue, leading to UAF.

Fix this by returning early in udptunnelnicdevicesync() if workpending is already set, preventing redundant work queueing.