Bluetooth: 6lowpan: use after free in disconnect_devices()

This was accidentally changed from list_for_each_entry_safe() to
list_for_each_entry() so now it has a use after free bug.  I've changed
it back.

Fixes: 90305829 ('Bluetooth: 6lowpan: Converting rwlocks to use RCU')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>