Bluetooth: hci_event: Add missing locking on hdev in hci_le_ext_adv_term_evt

Both hci_find_adv_instance and hci_remove_adv_instance have a comment
above their function definition saying that these two functions require
the caller to hold the hdev->lock lock. However, hci_le_ext_adv_term_evt
does not acquire that lock and neither does its caller hci_le_meta_evt
(hci_le_meta_evt calls hci_le_ext_adv_term_evt via an indirect function
call because of the lookup in hci_le_ev_table).

The other event handlers all acquire and release the hdev->lock and they
follow the rule that hci_find_adv_instance and hci_remove_adv_instance
must be called while holding the hdev->lock lock.

The solution is to make sure hci_le_ext_adv_term_evt also acquires and
releases the hdev->lock lock. The check on ev->status which logs a
warning and does an early return is not covered by the lock because
other functions also access ev->status without holding the lock.

Signed-off-by: Niels Dossche <niels.dossche@ugent.be>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>