Bluetooth: 6lowpan: Converting rwlocks to use RCU

The rwlocks are converted to use RCU. This helps performance as the
irq locks are not needed any more.
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>