Bluetooth: Move HCI command buffer user data into separate array

The HCI command buffers are the only ones with more than 4 bytes of
required user data. Move the user data into a separate array and do
the mapping with the help of net_buf_id(). After this, it will be
possible to reduce the default net_buf user data size from 8 to 4.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>