mac802154: fix data race and NULL deref on local->assoc_dev

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

mac802154: fix data race and NULL deref on local->assoc_dev

local->assoc_dev is shared between the association path and the
association-response worker without common synchronization.

mac802154_perform_association() stores the coordinator pointer and waits
for a response. Its timeout and error paths clear the pointer and return
to mac802154_associate(), which may then free the coordinator object.
Meanwhile, mac802154_rx_mac_cmd_worker() may observe the associating bit
and enter mac802154_process_association_resp(), which dereferences
assoc_dev.

The worker's bit test and the handler's pointer dereference are not
atomic with respect to cleanup. Cleanup can clear assoc_dev between them,
causing a NULL dereference, or free the coordinator while the response
handler still uses the pointer.

The recorded result is exposed to the same window. assoc_status and
assoc_addr are written by the handler but read by the association path
while the associating bit is still set, so a second response for the same
request - a malicious one, for instance - can replace them between those
reads and leave the caller with an incoherent status and address pair.

The response handler only needs the coordinator extended address.
Replace assoc_dev with a cached address, removing the pointer lifetime
dependency. Protect the cached address and the associating bit with a
dedicated spinlock. A READ_ONCE()/WRITE_ONCE() pair would not guarantee
an atomic __le64 access on all 32-bit architectures.

wpan_dev->association_lock cannot be reused here: nl802154_associate()
holds it across rdev_associate(), hence for the whole of
mac802154_perform_association() including the wait for the response.
A response handler taking that lock would only get it once the
association has already given up.

Reset the completion, publish the cached address, and set the associating
bit while holding the lock. The response handler takes the lock, rechecks
the bit and the cached address, records the response, clears the bit, and
only then completes the waiter. Thus cleanup cannot pass the handler
between its state check and completion, and the cached 64-bit value
cannot tear.

The handler clears the bit before completing, not the woken waiter:
otherwise complete() is issued under the lock and a second (e.g.
malicious) response can reacquire it before the waiter and replace the
result. So a wait that returns success implies the bit is already clear,
and the success and negative-response paths return directly. The
transmit-error and timeout paths still clear it under assoc_lock, which
serializes any racing response against the cleanup while the call returns
the error it already selected. Both paths snapshot assoc_status and
assoc_addr under the same lock.

Both users run in process context, so a plain spinlock is sufficient.
The lock is not held while waiting for the completion.