serial_core: Update buffer overrun statistics.

Currently, serial drivers don't report buffer overruns. When a buffer overrun
occurs, tty_insert_flip_char returns 0, and no attempt is made to insert that
same character again (i.e. it is lost). This patch reports buffer overruns via
the buf_overrun field in the port's icount structure.
Signed-off-by: Corbin Atkinson <corbin.atkinson@xxxxxx>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -2527,14 +2527,16 @@ void uart_insert_char(struct uart_port *port, unsigned int status,
 	struct tty_struct *tty = port->state->port.tty;
 
 	if ((status & port->ignore_status_mask & ~overrun) == 0)
-		tty_insert_flip_char(tty, ch, flag);
+		if (tty_insert_flip_char(tty, ch, flag) == 0)
+			++port->icount.buf_overrun;
 
 	/*
 	 * Overrun is special.  Since it's reported immediately,
 	 * it doesn't affect the current character.
 	 */
 	if (status & ~port->ignore_status_mask & overrun)
-		tty_insert_flip_char(tty, 0, TTY_OVERRUN);
+		if (tty_insert_flip_char(tty, 0, TTY_OVERRUN) == 0)
+			++port->icount.buf_overrun;
 }
 EXPORT_SYMBOL_GPL(uart_insert_char);