8250: fix possible deadlock between serial8250_handle_port() and serial8250_interrupt()

Commit 40b36daa introduced possibility that serial8250_backup_timeout() ->
serial8250_handle_port() locks port.lock without disabling irqs, thus
allowing deadlock against interrupt handler (port.lock is acquired in
serial8250_interrupt()).

Spotted by lockdep.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/8250.c

@@ -1334,8 +1334,9 @@ static inline void
 serial8250_handle_port(struct uart_8250_port *up)
 {
 	unsigned int status;
+	unsigned long flags;
 
-	spin_lock(&up->port.lock);
+	spin_lock_irqsave(&up->port.lock, flags);
 
 	status = serial_inp(up, UART_LSR);
 
@@ -1347,7 +1348,7 @@ serial8250_handle_port(struct uart_8250_port *up)
 	if (status & UART_LSR_THRE)
 		transmit_chars(up);
 
-	spin_unlock(&up->port.lock);
+	spin_unlock_irqrestore(&up->port.lock, flags);
 }
 
 /*