serial: expose buf_overrun count through proc interface

The buf_overrun count is only every written, and not exposed to
userspace anywhere. This means that dropped characters due to flip
buffer overruns are never visible to userspace.

The /proc/tty/driver/serial file exports a bunch of metrics (including
hardware overruns) already, so add the buf_overrun (as "bo:") to this
file.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1786,6 +1786,8 @@ static void uart_line_info(struct seq_file *m, struct uart_driver *drv, int i)
 			seq_printf(m, " brk:%d", uport->icount.brk);
 		if (uport->icount.overrun)
 			seq_printf(m, " oe:%d", uport->icount.overrun);
+		if (uport->icount.buf_overrun)
+			seq_printf(m, " bo:%d", uport->icount.buf_overrun);
 
 #define INFOBIT(bit, str) \
 	if (uport->mctrl & (bit)) \