Bluetooth: hci_ldisc: Clear HCI_UART_PROTO_INIT on error

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: hci_ldisc: Clear HCI_UART_PROTO_INIT on error

When hci_register_dev() fails in hci_uart_register_dev()

HCI_UART_PROTO_INIT is not cleared before calling hu->proto->close(hu)

and setting hu->hdev to NULL. This means incoming UART data will reach

the protocol-specific recv handler in hci_uart_tty_receive() after

resources are freed.

Clear HCI_UART_PROTO_INIT with a write lock before calling

hu->proto->close() and setting hu->hdev to NULL. The write lock ensures

all active readers have completed and no new reader can enter the

protocol recv path before resources are freed.

This allows the protocol-specific recv functions to remove the

"HCI_UART_REGISTERED" guard without risking a null pointer dereference

if hci_register_dev() fails.