wifi: mt76: mt7915: fix use-after-free bugs in mt7915_mac_dump_work()

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

wifi: mt76: mt7915: fix use-after-free bugs in mt7915_mac_dump_work()

When the mt7915 pci chip is detaching, the mt7915_crash_data is released in mt7915_coredump_unregister(). However, the work item dump_work may still be running or pending, leading to UAF bugs when the already freed crash_data is dereferenced again in mt7915_mac_dump_work().

The race condition can occur as follows:

CPU 0 (removal path) | CPU 1 (workqueue) mt7915_pci_remove() | mt7915_sys_recovery_set() mt7915_unregister_device() | mt7915_reset() mt7915_coredump_unregister() | queue_work() vfree(dev->coredump.crash_data) | mt7915_mac_dump_work() | crash_data-> // UAF

Fix this by ensuring dump_work is properly canceled before the crash_data is deallocated. Add cancel_work_sync() in mt7915_unregister_device() to synchronize with any pending or executing dump work.