tty/n_gsm.c: fix a memory leak in gsmld_open

If gsmld_attach_gsm fails, the gsm is not used anymore.
tty core will not call gsmld_close to do the cleanup work.
tty core just restore to the tty old ldisc.
That always causes memory leak.
Signed-off-by: xinhui.pan <xinhuiX.pan@intel.com>
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2366,6 +2366,7 @@ static void gsmld_close(struct tty_struct *tty)
 static int gsmld_open(struct tty_struct *tty)
 {
 	struct gsm_mux *gsm;
+	int ret;
 
 	if (tty->ops->write == NULL)
 		return -EINVAL;
@@ -2380,7 +2381,13 @@ static int gsmld_open(struct tty_struct *tty)
 
 	/* Attach the initial passive connection */
 	gsm->encoding = 1;
-	return gsmld_attach_gsm(tty, gsm);
+
+	ret = gsmld_attach_gsm(tty, gsm);
+	if (ret != 0) {
+		gsm_cleanup_mux(gsm);
+		mux_put(gsm);
+	}
+	return ret;
 }
 
 /**