Bluetooth: 6lowpan: avoid untracked enable work

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

Bluetooth: 6lowpan: avoid untracked enable work

lowpan_enable_set() allocates a temporary work item and schedules
do_enable_set() on system_wq, then returns to debugfs. The debugfs active
operation has ended at that point, but the worker still executes module
text and manipulates enable_6lowpan and listen_chan.

bt_6lowpan_exit() removes the debugfs files and immediately closes and
puts listen_chan. It has no pointer to the queued work item, so it cannot
cancel or flush it before tearing down the state that the worker uses.

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

debugfs enable write module exit
1. lowpan_enable_set() allocates 1. bt_6lowpan_exit() removes
set_enable work the debugfs file
2. schedule_work() queues 2. bt_6lowpan_exit() closes
do_enable_set() and puts listen_chan
3. the write operation returns 3. module teardown can continue
4. do_enable_set() later runs
against stale state

Run the enable state transition synchronously in lowpan_enable_set()
instead. The simple debugfs setter can sleep, and this file already handles
the 6LoWPAN control write synchronously under the same set_lock. Once the
setter returns, debugfs removal covers the whole operation and exit can no
longer race with an untracked work item.

Validation reproduced this kernel report:
BUG: KASAN: slab-use-after-free in do_enable_set+0x113/0x2e0
Workqueue: events do_enable_set [bluetooth_6lowpan]
The buggy address belongs to the object at ffff888109cb8000