n_gsm.c: add tx_lock in gsm_send

All the call to gsm->output should be in the tx_lock,
that could avoid potential race from MUX level. But
we have no tx_lock in gsm_send.

This patch is to add tx_lock in gsm_send.
Signed-off-by: Nxiaojin <jin.xiao@intel.com>
Acked-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -573,6 +573,7 @@ static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 	int len;
 	u8 cbuf[10];
 	u8 ibuf[3];
+	unsigned long flags;
 
 	switch (gsm->encoding) {
 	case 0:
@@ -602,7 +603,9 @@ static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 		WARN_ON(1);
 		return;
 	}
+	spin_lock_irqsave(&gsm->tx_lock, flags);
 	gsm->output(gsm, cbuf, len);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
 	gsm_print_packet("-->", addr, cr, control, NULL, 0);
 }