tg3: tg3_timer() should grab tp->lock before checking for tp->irq_sync

This is to avoid the race between tg3_timer() and the execution paths
which does not invoke tg3_timer_stop() and releases tp->lock before
calling synchronize_irq()

Reported-by: Peter Hurley <peter@hurleysoftware.com>
Tested-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -10903,11 +10903,13 @@ static void tg3_timer(unsigned long __opaque)
 {
 	struct tg3 *tp = (struct tg3 *) __opaque;
 
-	if (tp->irq_sync || tg3_flag(tp, RESET_TASK_PENDING))
-		goto restart_timer;
-
 	spin_lock(&tp->lock);
 
+	if (tp->irq_sync || tg3_flag(tp, RESET_TASK_PENDING)) {
+		spin_unlock(&tp->lock);
+		goto restart_timer;
+	}
+
 	if (tg3_asic_rev(tp) == ASIC_REV_5717 ||
 	    tg3_flag(tp, 57765_CLASS))
 		tg3_chk_missed_msi(tp);