Bluetooth: `BT_HCI_EVT_DATA_BUF_OVERFLOW` is unhandled

[alwa-nordic]

The handler exists, but it just prints a warning.

https://github.com/zephyrproject-rtos/zephyr/blob/5f3a69fada1a52700d9bc00d93a2b00d81c3ab57/subsys/bluetooth/host/hci_core.c#L557-L562

Deficiencies to address:

• This is definitely an error, not a warning. It informs the host that the host violated flow control.

  This event is used to indicate that the Controller’s data buffers have been overflowed. This can occur if the Host has sent more packets than allowed. The Link_Type parameter is used to indicate the type of data whose buffers overflowed.

• We should try to recover. Violating the flow control likely caused data to be dropped and leaves the host in an indeterminate state. Recovery could mean a Bluetooth reset.

[jeffwelder-ellenbytech]

Hitting this log message in my application.

We're using an nRF53 central/peripheral with LL_SW_SPLIT (mitigating missing disconnect issue in softdevice), and have come across this after a prolonged connections coming and going.

As a possible mitigation we added:

#ifndef CONFIG_BT_ASSERT
    LOG_WRN("Data buffer overflow (link type 0x%02x)", evt->link_type);
#else
    // https://github.com/zephyrproject-rtos/zephyr/issues/70495
    BT_ASSERT_MSG(false, "Data buffer overflow (link type 0x%02x)", evt->link_type);
#endif