tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()

tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()

Calling tty_find_polling_driver() can lead to uart_set_options() being
called (via the poll_init() callback of tty_operations) to configure the
uart. But uart_set_options() can also be called by register_console()
(via the setup() callback of console).

Take the console_list_lock to synchronize against register_console() and
also use it for console list traversal. This also ensures the console list
cannot change until the polling console has been chosen.

Signed-off-by: John Ogness <[email protected]>
Reviewed-by: Daniel Thompson <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Reviewed-by: Douglas Anderson <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>
Link: https://lore.kernel.org/r/[email protected]