n_tty: Don't wait for buffer work in read() loop

User-space read() can run concurrently with receiving from device;
waiting for receive_buf() to complete is not required.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -1724,7 +1724,6 @@ static inline int input_available_p(struct tty_struct *tty, int amt)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	tty_flush_to_ldisc(tty);
 	if (ldata->icanon && !L_EXTPROC(tty)) {
 		if (ldata->canon_head != ldata->read_tail)
 			return 1;