tty: serial: kgdboc: fix mutex locking order for configure_kgdboc()

Several mutexes are taken while setting up console serial ports. In
particular, the tty_port->mutex and @console_mutex are taken:

  serial_pnp_probe
    serial8250_register_8250_port
      uart_add_one_port (locks tty_port->mutex)
        uart_configure_port
          register_console (locks @console_mutex)

In order to synchronize kgdb's tty_find_polling_driver() with
register_console(), commit 6193bc90 ("tty: serial: kgdboc:
synchronize tty_find_polling_driver() and register_console()") takes
the @console_mutex. However, this leads to the following call chain
(with locking):

  platform_probe
    kgdboc_probe
      configure_kgdboc (locks @console_mutex)
        tty_find_polling_driver
          uart_poll_init (locks tty_port->mutex)
            uart_set_options

This is clearly deadlock potential due to the reverse lock ordering.

Since uart_set_options() requires holding @console_mutex in order to
serialize early initialization of the serial-console lock, take the
@console_mutex in uart_poll_init() instead of configure_kgdboc().

Since configure_kgdboc() was using @console_mutex for safe traversal
of the console list, change it to use the SRCU iterator instead.

Add comments to uart_set_options() kerneldoc mentioning that it
requires holding @console_mutex (aka the console_list_lock).

Fixes: 6193bc90 ("tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
[pmladek@suse.com: Export console_srcu_read_lock_is_held() to fix build kgdboc as a module.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230112161213.1434854-1-john.ogness@linutronix.de