hwmon: (occ) unregister sysfs devices outside occ lock

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

hwmon: (occ) unregister sysfs devices outside occ lock

occ_active(false) and occ_shutdown() unregister sysfs-backed devices while
occ->lock is held. hwmon_device_unregister() and sysfs_remove_group() can
wait for active sysfs callbacks to drain, and those callbacks can enter the
OCC update path and try to take occ->lock again. That gives the unregister
paths the lock ordering occ->lock -> sysfs callback drain, while a callback
has the opposite edge sysfs callback -> occ->lock.

This issue was found by our static analysis tool and then manually
reviewed against the current tree.

The grounded PoC kept the real unregister and callback carrier:

occ_shutdown()
hwmon_device_unregister()
occ_show_temp_1()
occ_update_response()

Lockdep reported the circular dependency with occ_shutdown() already
holding the OCC mutex and hwmon_device_unregister() waiting on the sysfs
side:

WARNING: possible circular locking dependency detected
... (sysfs_lock) ... at: hwmon_device_unregister+0x12/0x30 [vuln_msv]
... (&test_occ.lock) ... at: occ_shutdown.constprop.0+0xe/0x40 [vuln_msv]
occ_update_response.isra.0+0xb/0x20 [vuln_msv]
occ_show_temp_1.constprop.0.isra.0+0x23/0x40 [vuln_msv]
*** DEADLOCK ***

Serialize hwmon registration and removal with a separate hwmon_lock.
Under that lock, detach occ->hwmon and update occ->active while occ->lock
is held so concurrent OCC state changes still see a stable state, then
drop occ->lock before calling hwmon_device_unregister(). Remove the
driver sysfs group before taking occ->lock in occ_shutdown(), so draining
the driver attributes cannot wait while the OCC mutex is held. Also make
OCC update callbacks return -ENODEV after deactivation, so callbacks that
already passed sysfs active protection do not poll the hardware after
teardown has detached the hwmon device.