KillZramBackingDevice: Return immediately if backing_dev is none.

It's possible that CONFIG_ZRAM_WRITEBACK is y,
but userspace doesn't set the /sys/block/zram0/backing_dev,
so its value is 'none'.
It's the same with "CONFIG_ZRAM_WRITEBACK is not set".

Change-Id: I2df89ceee68e4685deef5113bada21be96779e9b
Signed-off-by: shisiyuan <shisiyuan@xiaomi.com>
diff --git a/init/reboot.cpp b/init/reboot.cpp
index ab0e48e..0e788e4 100644
--- a/init/reboot.cpp
+++ b/init/reboot.cpp
@@ -478,6 +478,11 @@
     // cut the last "\n"
     backing_dev.erase(backing_dev.length() - 1);
 
+    if (android::base::StartsWith(backing_dev, "none")) {
+        LOG(INFO) << "No zram backing device configured";
+        return {};
+    }
+
     // shutdown zram handle
     Timer swap_timer;
     LOG(INFO) << "swapoff() start...";