tty/serial: do not free trasnmit buffer page under port lock

LKP has hit yet another circular locking dependency between uart
console drivers and debugobjects [1]:

     CPU0                                    CPU1

                                            rhltable_init()
                                             __init_work()
                                              debug_object_init
     uart_shutdown()                          /* db->lock */
      /* uart_port->lock */                    debug_print_object()
       free_page()                              printk()
                                                 call_console_drivers()
        debug_check_no_obj_freed()                /* uart_port->lock */
         /* db->lock */
          debug_print_object()

So there are two dependency chains:
	uart_port->lock -> db->lock
And
	db->lock -> uart_port->lock

This particular circular locking dependency can be addressed in several
ways:

a) One way would be to move debug_print_object() out of db->lock scope
   and, thus, break the db->lock -> uart_port->lock chain.
b) Another one would be to free() transmit buffer page out of db->lock
   in UART code; which is what this patch does.

It makes sense to apply a) and b) independently: there are too many things
going on behind free(), none of which depend on uart_port->lock.

The patch fixes transmit buffer page free() in uart_shutdown() and,
additionally, in uart_port_startup() (as was suggested by Dmitry Safonov).

[1] https://lore.kernel.org/lkml/20181211091154.GL23332@shao2-debian/T/#u


Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>